Ethan wrote:
  You are probably thinking of the TM100s that shipped
with Kaypros
 and IBM 5150 PCs.  There's a (if _I_ remember correctly) model TM100-2
 and a TM100-4, SSSD and DSSD.  Someone here on the list, I think, has
 the ROMs for a Kaypro-4 to let it use DSSD drives. 
My recollection of common 5.25-inch drives (which could be wrong) was:
  Tandon   Teac
  full-ht  half-ht  specs                 use
  TM100-1  FD55-A   SS, 48 TPI            early PC single-sided, 160/180K
  TM100-2  FD55-B   DS, 48 TPI            PC 320K/360K
  TM100-3  FD55-E   SS, 96 TPI            not found in PCs
  TM100-4  FD55-F   DS, 96 TPI            not found in PCs
  TM200             SS, 500 Kbps, 96 TPI  not found in PCs
  TM201    FD55-G   DS 500 Kbps 96 TPI   not found in PCs, similar to but
                                         not interchangeable with the later
                                         1.2M PC/AT drives
           FD55-GF  DS 500 Kbps 96 TPI, 300/360 RPM  1.2M PC/AT
48 TPI drives are speced for 40 tracks, 96 TPI for 80 tracks.
The TM200 and TM201 were rare; I'm not sure if they ever actually made it
into volume production, although I had data sheets on them at one time.
I've always wondered what happened to the FD55-C and FD55-D.  The
FD55-GF is apparently still in production, but the other FD55 models
are long gone.  I've been looking for NOS FD55-B drives, but haven't
seen any in quite a while.