Use of 3.5" drives to replace 5.25" & 8-inch drives

Edited by Herb Johnson, (c) 2005; last changes Feb 20 2005

Documents, links

Old computer newsgroups often discuss converting old CP/M systems from 8-inch and 5.25-inch floppy drives, to 3.5" drives and media. Issues raised included support of FM or single density; finding old media; choices of formats. I decided these discusssions were worth preserving, as these issues come up over and over. For more technical information on floppy drives and floppy disks, go to our technical page on floppy drives and media. For specific discussions about converting from 8-inch, 5.25 inch, and 3.5 inch, look for links on this section of that page.


Jay Maynard posted in comp.os.cpm on Jan 22 2005:

" if I wanted to hook up a 3-1/2 inch drive to my S-100 system instead of the SA-800s that are on it now, would the WD 1771 talk to them and work? Yes, IBM 3740 format would be Just Weird on a 3-1/2 inch floppy, and I don't expect to read the data with a normal PC without special software, but this would represent one way to get data from the 8-inch system into, say, a P112 (with a little bit of progamming)."

This led to a discussion thread about creating standard formats for the use of newer drives on older systems. It's an old topic, but along the way there was informative discussion about the technical issues of single, double and high density media and drives; floppy controllers; and formatting. It also led Randy McLaughlin to propose some standard formats and to discuss tradeoffs and expectations about reliability and compatibility with several other comp.os.cpm regulars. Randy also moved the discussion to the "classic computer" maillists at

I've compiled, sorted and edited the discussion for ease of reading and future reference. The problem with discussion threads is that they rarely lead to either results or a useful permanent reference; and quick discussion can gloss over problems. I hope my editing and archiving efforts support both study and results.

My editorial changes and additions are either in []'s for edits and add-ins; or in italics (except the above paragraph) to name authors. While I retain copyright to this document, the individuals named have copyright to their posted as quoted, and have given me permission to quote them. Changes, corrections, and more info is appreciated. But, you may want to post in the appropriate maillist or newgroup any responses in the original thread. This is an archive and reference, not a replacement for those discussions.

See also the corresponding edited discussion on the same subject, from the "cctech" and "cctalk" maillists of, on my site in the file named: 8inch_cctech.txt

Randy McLaughlin is continuing this discussion, and starting an effort to create some standards, on a "disk drives" page on his Web site. (Note: As of mid-2006 Randy does not appear to be active.)

Herb Johnson

Table of contents:

8-inch drives vs. 3.5's
On possible standards
Problems of single density/FM
diskette/drive reliability

8-inch drives vs. 3.5's:

Randy McLaughlin comp.os.cpm Jan 27 2005:

I also believe in 8" disks as a great media.  My point is and always has 
been that keeping 8" drives working is a dead end road.  Sooner or later 
something fails that can not be repaired or replaced.

I take two roads to preserve my CP/M systems:  Buy spare drives when I can 
find them for the right price.  Use 3.5" drives.

I find that 3.5" drives have several benefits:  Easier power requirements, 
cheap, plentiful, easy to transfer data to PC's (I have a PC that I have an 
8" drive on but 3.5" is easier than dragging out another PC), takes less 

As I said I like 8" drives and still have several, some are not functional 
but I keep them for parts or to repair later.  The real point is 3.5" works 
and we must bring our systems into the 21st century.

> Herb johnson posted:
> However, I'll say up front that writing a unique floppy disk format is
> a TERRIBLE idea. Why create a disk that only one system in the world
> can read? A search of this newsgroup's archive will inform you of the
> troubles owners have, just with disks that only one BRAND OR MODEL of
> system can read!

Jay Maynard replies: [using a 3.5" drive on my 8-inch based system]
... is intended as a temporary expedient until my P112 arrives, and as a
supplement to my 8-inch drives on my S-100 system. I'm having great
difficulty with keeping them running, as they seem to be hrowing CRC errors
more often than not on the same disks that, other times, read correctly.
I've also got one drive that seems to insist on dropping ready at the most
inconvenient times.

Yes, unique formats suck. They suck less than not running at all.

On possible standards

Randy McLaughlin, comp.os.cpm Jan 27 2005:

As Herb stated we must not all go in a thousand directions.
We should come up with a standard.
I sent an email to Howard Harte recomending changing the disk format for 
the [IMSAI] SuperIO.  The SuperIO boots from Flash ROM and as such 
needs no reserved tracks, I recomended reserving two system tracks to be 
compatable with systems that boot from floppy.

There are three physical formatting standards I recommend:

32 128 byte FM sectors for 3.5" HD media.

9 512 byte MFM sectors for 3.5" DD media.

18 512 byte MFM sectors for 3.5" HD media.

These are simple standards easily used by CP/M systems.

I personally have disks going back to the 70's and have had a lot more than 
2% failure to read (most are readable but maybe 20% are not).

Harold Bower replied, 1/27/05:

I would recommend a different set of DD formats.  I would suggest the
default formats used for the Ampro Little Board, SB-180, SB-180FX,
Yasbec and ON!  The same set of formats were used on all for 3.5" DD
drives (same as 80-track 5.25) as 5-1025 byte sectors/track (sectors
numbered 1-5) for single-sided and (sectors numbered 16-20) for
double-sided with skew recorded during format (normally 2) and
sequential readback.  for 40-track 5.25" drives, 10-512 byte
sectors/track (sectors numbered 1-10) single-sided and (sectors numbered
16-25) double-sided with the same convention for skew.  Two tracks were
reserved for boot tracks in all formats.

I included this format series in mods I did to the P112 as well and
included a format that has been very robust on SB180, SB180FX, YASBEC
and P112 for 3.5" HD drives of 11-1024 byte sectors per track for a
total capacity of 1.76 MB per diskette.

"Randy McLaughlin" wrote, and Hal responded, on Jan 28th & 29th:

>> Randy: I agree that the disks can be formatted to maximize space.  The
>> problems, as I see it are:
>> Many systems such as Tarbell do not already have 1K support.

Hal: Good point, but how many Tarbell systems are still operational vs SB180,
SB180FX, Ampro Little Board, On!?

>>I would guess there are many more Tarbell systems running than the total of 
>>the above mentioned systems.  Tarbell disk controllers were and are 
>>extremely popular.

>> On the systems Like Tarbell 1K support can be easily added but on
>> stock CP/M v2.2 systems it reduces the TPA by more than 512 bytes.

Only if the system does not now support 1024-byte sectors.  From Cam
Cotrill's any my work on ZSDOS back in 1987/8 and B/P Bios 90-92 work,
we found very few that did not.  In any event, would you be willing to
go from, say a 58k CP/M configuration to a 57.5k system to gain 80k more
storage on an 80-track DD disk?

>>Here we are talking about extremely small numbers either way.  Any system 
>>that supports 1K sectors have more than 512 bytes less TPA than systems 
>>which support only 512 byte sectors.  The 80K takes into account the entire 
>>disk, it is a little less when you take out the system tracks.
>>I personally have no special preference for sector size, 1K is fine with me. 
>>The SuperIO is a product of Howard Harte and before making changes in his 
>>CP/M implementation I will get his input (I have sent him a private email 
>>asking for his opinion).

>> By using 512 byte sectors you only "need" to fill the directory area
>> with 0E5h's to convert a PC disk to a CP/M system.

That is a valid point, but probably not as critical on Linux systems as
DOS/Windows where the OS seems locked heart-'n-soul into 512-byte

>> Disk formats can be optimized for each environment but it misses the
>> point of standardizing.  All of the newest CP/M implementations such
>> as updates to CP/M-86 & the Imsai series two have been following the
>> PC lead and have been using 9/18 512 byte sectors.

What standard?  The only "Standard" is the IBM 3740 which is 8", 77
track single sided, 26-128 byte sectors per track FM.  There was a later
modification (forget the number ATT) that added Double-density and
double-sided, but again only for 8" disks.  Where is Joe Wright when we
need him?  He wrote the BIOS for the Davage DiskMaker II which was the
cadillac of conversion processors (I had one, and hacked the Bios after
talking with him).

If we consider market share as a "defacto standard" then it makes more
sense to look at the operational systems out there, and again, I venture
to suggest that for bootable disks, the Ampro formats would prevail with
emulation being used (either a la the Ampro E-Disk or with the emulation
table in B/P Bios) to use the PC-style 3.5" disks as an exchange medium.

I adapted B/P Bios to the SuperIO and still have it up on the OLD
version of the board, but have not heard anything from Howard since
sending him the image, nor have I seen it available for D/L (so I guess
I'll add it to my page when I update it).  In that system, I boot from
the CP/M3 disk, then switch directly to the Ampro-formatted diskettes
from which warm boots and all work can then proceed, including the IBM
format emulation.

>> It is obvious that more discussions such as this need to be expanded. 
>> I'd be happy to change the SuperIO BIOS to 1K sector sizes if that is
>> the direction people want to go, it is more disk efficient.

If you were to adopt the Ampro-"standard" formats, the 2-track boot
sector reservation is already there, the skew factor of 2 (normal, but
changeable) is there, and you would have complete interchangeability
with a great many other systems, and an extra 80k bytes per 3.5" DD
disk, and 40k per 40-track DS or 80-track SS disk.  The formats have all
proven remarkably robust and have plenty of tolerance slop in them to
accomodate loosey-goosey hardware.

>>Randy:Also referring to a previous post why the odd sector numbering on the second 
>>side?  I would always prefer to start sector numbering with 1.
>>In truth I was hoping this discussion would have more people involved to get 
>>a true consensus.  We have an opportunity to direct the transfer to a common 

Hal: The "odd" sector numbering is not just for the second side, but for the
entire disk.  The algorithm used in the Ampro Little Board, SB180,
SB180FX, On! etc was something like:
 1. Home heads
 2. step in two steps
 3. Read Sector ID from side 0 (first side .. bottom)
   a. Track number was 2, drive was 80-track
      if Track number was 1, drive was 40-track, so set flag to
       (allows 40 track disk to be used in 80 track drive .. carefully!)
   b. If sector number was < 16, then drive was single-sided (1..5,
1..10, 0..10, etc)
      else assume drive is double-sided

3.5" drives (DD) were treated the same way as 5.25" (DD) units so this
simple algorithm allowed a wide range of complements to be handled
simply and efficiently.  Special handling was often included to allow
Kaypro disks (512-byte sectors numbered 0 .. 9) to be detected along
with the Ampro 1 .. 10 numbers in 40-track drives by doing an explickt
read for Sector 0 if step 3 indicated a 40-track drive.

Others expanded this in various ways.  For example, the author of XBios
for the SB180 started sector numbering 80-track double-sided formats
without boot tracks (intended for data-only disks) at 27, neatly
avoiding the highest number normally used in that day of 26 in 8" and
down. In B/P Bios, Cam and I chose an offset of 30H (start at 49
decimal) for 5.25" HD formats and 40H (start at 65) for 80-track HD
formats (where the Bios automatically switches density while logging
disks in) while Tilmann Reh starts HD formats at 1 in 3.5" systems.  If
you are going to allow mixed drives in systems without locking users
into specific configurations, then implementing some algorithm and
implementing non-conflicting formats might be a bit more usable.

I too would like to see more people involved in this, but I doubt if
there will be a single "common format".  Anyone???

Problems of single density/FM

Randy McLaughlin, Jan 22:

To replace an 8" drive with a 5.25" HD drive is a snap, to hook up a 3.5"
drive requires modifying the BIOS.

The data transfer rate are compatible but the rotational speed of the 3.5"
drive is slower allowing more data to be written.

There are lots of technical differences in the different types of drives.
Herb Johnson has been good enough to collect and post some info: [note from 
Herb: I've changed the link referenced below:]

The modifications to the BIOS/format program are not too great, mainly
increase the tracks to 80 use both sides and change the SPT (sectors per
track).  I've never done it with a 1771 so I am not sure of how many 128
byte FM sectors the average 3.5" drives can handle.

To use 5.25" drives is much easier and does not require any software

Luckily the BIOS and format program for the Tarbell controller is available
and easily modified.

After checking with [TEAC FD-235HF 3.5" drive docs at:]

I found the 3.5" drives can be formatted FM (1771) as follows:

128 bytes per sector 32 sectors per track 655K per disk.
256 bytes per sector 18 sectors per track 737K per disk.
512 bytes per sector 10 sectors per track 819K per disk.

Unless the BIOS is already handling larger sector sizes stay with 128 byte

Additional, from Herb, unposted:

[The TEAC document specifically says that an FM data stream at
250Kbits/second on an 80-track HD diskette has a low bit density (8717 bpi) but
the same FLUX density (17,434) as MFM - as Randy posted. The document has the
format schemes Randy mentions above, for "2MB" HD diskette format. For DD media at
"1MB" format, with a flux density of 8,717, the document specifies FM use at
125Kbits/second, about 400Kbyte capacity; and MFM use at 250Kbits/second for about 
800Kbytes capacity. Disk rotation speed is specified at 300 RPM. Write precompensation,
a kind of hardware adjustment for reading and writing data frequency effects, 
is listed in the document as +/-125ns for "2MB"; "0 +/-125ns" for "1MB".]

CF Falconer replied:

> Why not cut the tracks to 26 sectors with wider gaps, and now you
> have a track by track clone of SSSD and DSSD 8" drives, with
> 256/512 k capacities and 4 extra tracks per side.  CP/M can easily
> read those extra tracks, but writing them requires a driver.  So
> they can best be used for mounting r/o files. 

Randy: No matter what, you need to modify the software even if it is only
the format program. [Otherwise,] the tracks can be formatted to 32 sectors and the BIOS
will ignore the extras. The format I described works even if only the first 26
sectors are used by the BIOS. With only the slightest change in the BIOS,
you can handle near 650K of data (leaving room for the boot tracks).

It would be best to test sector interleave and skewing as well as step
rates. The format program will usually fail because the 3.5" drive rotates at 300
RPM. I happen to know that [the controller in question] is a Tarbell controller
and the source code to everything [in it] is available. 

[Herb: responses to Randy follow in the middle of the "format" discussion below.]


Glen Herrmannsfeldt posted on Jan 23rd:

> Will [the format program really fail?]  After the last sector 
> there should be a gap until
> the index.  If it times out before it gets to the index it
> would fail, but I don't know that they do that timing, or at
> least not to that precision.  Do they really do that? 

Randy: Maybe not every one but Cromemco & Compupro won't, supposedly Tandy M2 &
derivatives won't either.

It would be worth trying it and making a list of what has to be done to get
it working when possible.

Later versions of the Cromemco disk formatting program ( won't even
format 8" disks with their faster Z80 CPUs.  They have a software timing
loop that requires that the 8" drive RPM be extremely close to 360 (less
than +/-5) and the CPU be running at 4mhz.

I have never seen that in any other formatting program.  I have a couple of
XPU's (5mhz Z80's) and they have notes from Cromemco stating that you can
not format 8" disks with them, I've tried and they don't. 

Glen Herrmannsfeldt posted in reply:

The disk formatting programs that I did the most work with were
the OS/9 routines for the 6809 based color computer.  That used
the WD1793, where formatting a track required sending a byte to
the 1793 for each byte on the track, including gaps.
First it makes the track image in memory, and then runs the loop to
write that to the 1793.  The loop isn't very much faster than
the data rate, and not fast enough for twice the data rate of 8" MFM disks.

(500,000 bits/second, compared to 250,000 bits/second for 5.25").

For the write track command, data is supposed to be supplied for
gap4 (after all sectors) until the interrupt comes with the
next index pulse.  The numbers in the manual or for exactly
360RPM with no extra for slow drives or fast crystals.
The gaps are to allow for a reasonable speed tolerance on
drive motors.

For any write operation if the data register is not filled in time,
X'00' will be written and the lost data bit set in the status register.

Best would be for format programs to continue writing zero until the
index, pretty much what the manual says, but they might not do that.
Second easiest is to ignore the lost data bit.

I believe (but don't have the data book out) that the Intel/NEC
controllers don't require all the characters for the gap to
be written to the data register, so formatting would be a little

Barry Watzman responded on Jan 24th:

Glen, your information is correct, but you solution has a problem:  it can
cause the system to hang indefinitely with no means of recovery other
than a reset or power off.

For this reason, almost all disk formatting programs will provide only a
limited number of bytes following the last sector.  That number is
calculated to be "generous" in case of a slow turning drive, but not
"infinite".  And, frankly, even a generous value will almost always "run
out" and declare an error if the drive is turning at 300 rpm instead of
an expected 360 rpm.

This is very easy to deal with IF YOU HAVE THE SOURCE CODE, but it's the
principle reason that format programs will fail if you sub a 3.5" drive
(or even some 5.25" drive) for an 8" drive.

Jay Maynard's posts in reply to Randy's and others:

> Randy: It would be best to test sector interleave and skewing as well
> as step rates.

I suspect the 3-1/2 inch drive would easily be able to keep up with the
1771's 6 millisecond maximum step rate.

> The format program will usually fail because the 3.5" drive rotates at 300
> RPM [versus 360 RPM].

This is a 20% change in speed. The 32 sectors/track format is 23% more. What
accounts for the difference?

> With only the slightest change in the BIOS you can handle near 650K of data
> (leaving room for the boot tracks).

Yeah, and the boot tracks get bigger as well (though, there, I'd probably
have to leave it at using only 26 sectors unless I wanted to rewrite the
boot loader and SYSGEN).

> randy: I happen to know that it is a Tarbell controller and the source code to
> everything is available.

Jay: Yup. It's an original Tarbell SSSD, and the mods are straightforward. The
1771 shouldn't complain about the longer tracks.

One other advantage to doing it this way: Copying disk images back and forth
to SIMH/Altair[, the Altair simulator] gets simpler, since the tracks don't
need to be padded. 

Looking at the 1771's 3740 format specification, it looks like it wouldn't
be hard to do either 26 or 32 sectors per track on a 300 RPM drive.
Formatting with 26 sectors involves wasting 1042 bytes per track, either by
increasing the block gap from 33 to 73 bytes and increasing the index gap
from 320 to 322 bytes, or by increasing the index gap from 247 to 1087 bytes
and leaving everything else the same. I haven't looked closely at DFOCO's
track formatter, but if it uses the method recommended by WD (writing 26
sectors, then writing the index gap until the 1771 says "enough!"), then
there may be no need for software changes at all. This might not be so good
for performance, but I'm not sure it would make that big a difference.

Formatting with 32 sectors simply involves writing 32 instead of 26 sectors,
then reducing the index gap from 320 to 234 bytes. This would probably work
well (and may require only minimal changes to DFOCO, and only a new DPH/DPB
for the 3-1/2 inch drive). 

> [herb:] Also, I saw that the 1771 FDC chip was mentioned. That chip does not
> have an on-chip data seperator - it is only good for SINGLE density
> unless you have a seperate data seperator circuit or chip. Details of
> such circuits? - check various S-100 manuals on FDC cards with 1771's.
> I have a few such manuals, as do others.

My system has an original Tarbell single-density controller, complete with
external [data] separator.

>Randy: FM & MFM have the same "density" when you look at the magnetic fluxuations.
>FM uses a clock bit between every data bit while MFM uses a coded data bit
>with no clock.  As such the 3.5" diskette sees them as the same "density".
>Please note I am loosely refering to density, limiting it to the flux
>density of the diskette.
>This is why it is OK to use HD diskettes written in FM (single-density),
>this applies to all disk sizes (3.5", 5.25", 8"). 

JGH (J.G. Harston), on Jan 30th: 
>>Yes, as long as you don't try to write at HD rate onto DD disks, or at
>>DD rate onto HD disks. (Don't tape over or punch a new HD hole.)
>>Writing FM on HD media gives you the same capacity as writing MFM on DD
>>media, but it's a bit of a waste of the HD media. A controller than can
>>write to HD media will be able to do MFM, so you may as well actually
>>use MFM when using HD media.

> Randy: A controller than can write to HD media will be able to do MFM, so you may
> as well actually use MFM when using HD media.

Jay Maynard: Not if you're only using an FM-capable controller. This discussion started
originally because I asked about hanging a 3-1/2 inch drive on my S-100
system, with a 1771 controller - which only does FM. 

[later, in private remarks, J.G. Harston discussed his experiences with using 3.5" DD media and drives to replace 5.25" drives. Harston has a substantial amount of technical notes on the Web at - Herb]

[J.G.Harston:] My experience is using and writing filing systems (BDOSs in CP/M parlance) and replacing 5.25" drives with 3.5" and/or 3" drives on Acorn BBC and Masters [computers] with 8721, 1770 and 1772 [floppy] disk controller [chips], with disks formatted FM (8271/1770/1772) and MFM (1770/1772). Early BBCs had PCB links to connect to 8" drives, but I have never used them.

I found that with every machine I tried a 3.5" or 3" drive could be plugged in in place of a 5.25" drive with no change. One filing system using the 8271 needed a small patch to change timing parameters, I suppose the equivalent of a CP/M BIOS patch.

The only requirement was that DD/blue/single-hole 3.5" disks had to be used. HD/black/two-hole 3.5" disks would fail either when formatting or later. DD disks would successfully format to 2*80*10*256 FM -> 400k 2*80*16*256 MFM -> 640k 2*80*9*512 MFM -> 720k as used by DOS 720k 2*80*10*512 MFM -> 800k 2*80*5*1024 MFM -> 800k

Later RISC OS machines used 82710 and later controllers and could manage HD formats including 2*80*10*1024 MFM -> 1600k.

diskette/drive reliability

Randy, earlier post:

I personally have disks going back to the 70's and have had a lot more than 
2% failure to read (most are readable but maybe 20% are not).

From: Bill Hutchinson,  27 Jan 2005 

...Maybe also worth considering [is] how long
any of those 3 1/2'' disks will be readable. I can still read
most of my 25+ year old 8'' disks, but many 3 1/2'' ones
that I formatted/wrote, and could read back, only 1 to 3
years ago are now unreadable (on x86 machines).

Don't think it's all due to 765 vs WD-17xx controllers.

Who used to say things weren't properly backed up
until it was on 800BPI 1/2'' tape? Or, was it 1600BPI?

from: Randy McLaughlin in reply to Bill:

Archiving is what you are referring to.  Archiving deserves its own thread,
what I do is try to get the info to my PC and do archiving from there.

If you want to store data for 25 years and then try and read it I do not
recommend floppy disks, I have seen people try that with magnetic tapes
(1/2" as well as cartridge) with disastrous results.

I too have been able to read very old disks at least sometimes, especially
FM.  The issue is bit drift the magnetic flux density is quite a bit lower
in 8" media vs. 3.5" media, as such a little bit drift on 8" media will
cause fewer problems than the same drift on 3.5" media.

The real point is keeping 8" drives working is an issue as well as copying
data between PC's and CP/M systems is easier.

from Jay Maynard on Jan 27:

I've had pretty good results reading 9-track mag tapes from the late 60s
through the early 80s. I've had problems with tapes from the mid-80s, but
not because of bit deterioration: the tapes (principally Memorex MRX V) have
a problem where the binder migrates to the tape surface and makes the tape
stick to the read/write head.

I only had a couple of my 8-inch SSSD disks fail to read, and they're from
the early 80s.

> Randy said: "The real point is keeping 8" drives working is an issue as well as copying
> data between PC's and CP/M systems is easier.

Yeah. I still don't know if my current problems are due to my SA-800s or my
Tarbell [S-100 floppy disk controller, presumably FD1771 SD format - Herb].

from Barry Watzman on Jan 27:

For what it's worth, lots of us have 30 year old floppies from the 70's,
and my experience is a read success rate of about 98% (on hundreds of
discs), and most others reported similar results when we discussed this
before, about a year ago.
Herb Johnson posted on Feb 10th:

What interests me more about this thread is the issue of reliability
and long-term use, of either old schemes or new schemes. Randy
suggested that was an "archival issue...for another thread",. But any
standard has to be RELIABLE, else why bother to write disks that will
become flaky?

Randy replied:

> My reference to archival issue is simply that storing any
> magetic media for
> a long period of time is an issue.  The problem of bit drift is an
> issue, it applies to any magnetic storage.

Herb responded:

I'm not talking about "bit drift". I've mentioned this in other posts.
I'm talking about RELIABILITY - writing to disks that you can expect to
read, months and years later. And, PORTABILITY - reading disks from
System A on system B. I'll explain again, in a couple of ways, to make
the point.

You (among others) propose putting new drives on old systems, to write
to new disks. What if the old systems can't RELIABLY write or read new
disks? What if new systems can't read the new disks? What if these
problems don't occur now, but later, after what you call "bit drift"?

It's already been discussed that single density is an issue. The oldest
systems can't write anything but single density: but not all new
systems can also read single density. Some have said that's a marginal
concern, there aren't many systems that old. Point being - and you,
Randy, and others have said this -  it's the OLDEST, 8-inch SD systems,
which are in greatest need of some kind of drive upgrade.

The issue of *reliability* is about whether you will lose bits sooner
than you think, for reasons a little less clea? Like, pushing the
drives or media too close to their design limits? Disks and media are
ANALOG devices, their data depends on detecting signals on magnetic
media, and putting them on that media. A drive and disk and format may
work today - but will it work a year from now?

The issue of *portability* is: Will it work on many different drives,
and disks, and controllers? If you write to a drive or media or
controller on the margins of its design, that means that some OTHER
drive, or diskette, or controller design may not be able to read it.
The classic example is writing 40 track diskettes on 80 track drives.
The 80-track track width is NARROW, you will not completely overwrite
the old, wider 40-track data. Result: errors, now or later as the
magnetics degrade.

I think these examples and situations are reasons to be cautious. Yet,
as you (Randy) found, one drive's data sheets made the case for
FM/Single density more reasonable than I thought. It helps to have some
facts in hand.

Randy, one reason I'm compiling this discussion, is that it is easy to
put aside difficulties because, by and large, most systems will be able
to accept 3.5" drives without too much fuss, at least at first. But the
question remains: how MUCH fuss? what are the "gotchas"? what are the
limitations? When will they be reached? I think the limits will be
about the oldest systems, versus the newest media and drives. That's
what I'm paying attention to.

I'll say it again and again: "what use is a diskette that can't be read
by another system?" A premise which leads to that question, is the fact
that systems DIE, and a one-only-system disk, a just-for-now solution,
will become a ZERO-only system disk at the wrong time. The virtue of
Randy's idea of "standards" avoids that difficulty, at least at the
level of sector and track.

That's all I have time for right now. I already spent half a day
reviewing the prior discussion, sorting it out to be EDITED and
archived to be useful. It's not helped, that the discussion is over two
other maillists. Funny, Randy at one point said there was not much
response - that was several days ago.