How can AIX tell 1/4" cartridges apart?

[Dave Olson]

In <211.27d642c4 at vax1.cc.lehigh.edu> lusgr at vax1.cc.lehigh.edu (Steve Roseman) writes:
| How does AIX tell what kind of 1/4" cartridge is in the drive?
| 
| The question may be trivial, but I wasted at least 2 hours with the problem.
| I grabbed 2 fresh 6150 (150MB) tapes from the box to backup my 3003 release.
| I then proceeded to waste about 2 hours trying to write on the first tape, but
| kept getting errors whenever I tried to write on /dev/rmt0.  The errors were
| immediate, not write or media failures.  I rebooted, powered everything off,
| nothing helped.  Diagnostics passed with no problems.  On a whim, I tried
| writing to /dev/rmt0.4 (low density), and it worked.  Finally, I tried the
| second tape, and no problems at all.  First tape again, rmt0.4 works, rmt0
| doesn't.  I then happened to notice that on the back of the failing tape,
| included in a mass of numbers it said 600A, and the good one said 6150.  Ah,
| the manufacturer put the wrong label on the tape, so I complained, etc. 

The drive detects the difference by the hole pattern at BOT.  It sounds like
AIX didn't detect it (or at least report it with a unique error) at all.

The drive itself will refuse to write the wrong type of tape (normally
QIC24).  Now the interesting thing is that QIC120 (i.e., 600A) tapes are
normally writable on a QIC150 drive...  So what is the scoop; is IBM using
a QIC150 only drive, or are they programming the drive to run in only
QIC150 mode?