How can AIX tell 1/4" cartridges apart?

[Steve Roseman]

Today's Trivia Question:

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. 

So, how did AIX know?  I can't see any differences in the cartridges, so I
presume it has something to do with the little dance the tape does when put
into the drive.  The manufacturer's rep said by the tape oxide color, but I
don't believe that. 

Thanks for any responses,
Steve

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Steve Roseman 
Lehigh University Computing Center 
LUSGR at VAX1.CC.Lehigh.EDU