Are old SCSI tape drives not all created equal?

[js at cimmeri.com]

   Hi, folks.

   I'm experimenting with various old 
SCSI tape drives to see which
will work with my PDP-11/34 with an 
Emulex SCSI card.

   To my surprise, not all SCSI tape 
drives are created equal.  I
was under the mistaken assumption that 
all SCSI tape drives would
pretty much be abstracted the same way 
by the SCSI interface.

   Drives that are working fine are:

   - Archive Python DAT
   - Exabyte 8mm
   - Cipher 995 9 track
   - any of the DEC branded DLT drives


   Drives that aren't recognized are:

   - Teac MT-2ST (this drive seems 
completely nonstandard -- can't
                  even use it as an ASPI 
drive under MSDOS)

   - OnStream Advanced Digital Recording 
(ADR) SC-30 (doesn't
                  work under MSDOS / 
ASPI either, but does work
                  under Windows 98(!) 
with a special driver.
                  Won't work under WinXP 
with same driver.

   - Quantum branded DLT drives (work 
fine under MSDOS / ASPI)




   Question: So, even though some tape 
drives physically have a SCSI
interface, are they different in some 
other way such as to require
special software to use them?   Or maybe 
there were different
SCSI standards?  Or is the standard 
simply imperfect?

   Any commentary, appreciated.  Thank you.

- John