Drives typically become unavailable (as in Show Libr output) due to hardware
errors, as in a tape stuck in the drive, or the drive having been rendered
unavailable through operating system command, or marked unavailable through
library manager, or someone erroneously turned off some library hardware for a
while.  There is usually an Activity Log entry when ADSM runs into the
condition; but maybe not.  There may be an 'errpt' entry, or the library may
have its own log or evidence of an Intervention Required condition.
    Richard Sims, BU