First of all, the problem is with the autochanger, not the drive (note the
ANR8300E says "library" (ie autochanger).
The '207' is a general error saying
the mount can't be serviced, and the FF's in the other fields mean that there
was no sense data available that would give you more specifics.  So it is very
hard to say what's happened.  However, the 315 means "destination is full", ie.
either the drive on a Load, or the target slot on a dismount.  This may give
you a hint.  For instance, the mount may have reported a failure, but the robot
actually loaded the drive and the volume got left there.  All subsequent loads
then fail with "destination full".

It may help just to monitor the device when this is going on, if possible, to
get an idea of what is going on.  You should also check if there are microcode
updates available for the device.  I would call IBM service if you can't
resolve it any other way.  They'll probably ask for traces/etc.

Craig Bell