Fred Cisin cisin at
Wed Feb 3 19:09:21 CST 2021

> ideally, you'd use a 96tpi drive on 48tpi and microstep the head 
> positioner. you still have the problem of head clog.

Some "stupid ideas":
In line with trying to minimize possible damage during analysis of the 
format, try:
    Use a 96tpi drive.

    Try to read cylinder 72.  MOST OS's are aware of the lower reliability 
of inner tracks, and will try to write the data to the outer (lower 
numbered tracks)  Therefore, if the oxide comes off on your head, then it 
is less likely to be what you needed.

    See if you got a valid read.  USUALLY (not always), that cylinder will 
have the same number of bytes per sector, sectors per track, and heads per 
cylinder as the other tracks.

    If the track read identifies itself as track 36 (look at the CHRN 
fields if there is an IBM/WD style sector header) then it was presumably 
written on a 48tpi drive.
If it identifies itself as track 75, then it was probably written on a 
100tpi drive.  Switch to a 100tpi drive (MicropolisII, Tandon TM400-4M, 

Then try cylinder 36.
If the disk had been written on a 48tpi 35 track drive (SA400/SA390 
used on TRS80 and Apple), then cylinder 72 of a 96tpi/cylinder 36 of 48tpi 
would not have been used.  If cylinder 72 does not read, but 36 gives a 
valid read that identifies itself as track 18, then it is most likely 
from a 35 track 48tpi drive, such as SA400.

Watch out for re-used disks.  If a formatted 5150 disk was reformatted by 
a single-sided machine, then you will have the single sided format on 
side A, and PC format on side B!
Similarly, a used 40/80 track disk reformatted by a 35 track machine will 
have different formats from the residual formatting on the high tracks.

Every even numbered cylinder of a 96tpi drive corresponds to a cylinder of 
a 48tpi drive.
Every multiple of 24 cylinders of a 96 tpi drive corresponds with a 
multiple of 25 cylinders of a 100tpi drive.
If you succeed in reading the track identification (CHRN in sector header 
in IBM/WD formats), then you have a clue of the tpi of the source drive.

If you managed a successful track read, then knowing the tpi, number 
of heads, bytes per sector, sectors per track, will let you avoid some of 
the otherwise unsuccessful read attempts (such as unnecessary revolutions 
of the media trying to read between tracks on 48tpi)

Is it worth the effort to modify the drive?
Reading both heads at the same time would halve the number of revolutions 
needed of deteriorating media.

I had some 8" diskettes for which I could get a successful read IFF I 
pushed gently on the side of the head with my finger.  I always intended 
to try to add a micrometer lead screw to the head positioning, but never 
got around to it.  Presumably, if the amount of offset from "correct" 
position is determined, it would probably be the same for most/all disks 
written with the same source drive.

Howzbout analog positioning?  Amlyn claimed to use "analog positioning 
with phase locked loop feedback" to find the tracks.  Did they?

If you have some semi-skilled assistants (interns, grad students, etc.) 
then some of the catch up of the backlog could be handed off to them for
   disks that aren't as likely to be damaged by a read attempt
   disks with a known/presumed/likely format
If you have a group of workstations (and a swivel chair), you could then 
step-in only when the assistant(s) encounters a problem.

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