Inspect your server activity log;  there's probably better info, there --
like a tape mount request that didn't get satisfied within the "mount like a 
tape mount request that didn't get satisfied within the "mount wait"
time on the device class of the storage pool your
management-class/copy-group points to.  I've seen this happen where the
max-scratch was set too low, or the TDP node name was trying to use multiple
mount-points (when the node was registered for less than the request)...
WAD.

Also, most backup agents (via the API) have gotten smart enough to send the
estimated "glob" size, which can easily be used to send the data to the
nextstg pool (usually, tape).  We set max-size on most disk pools, and we
use scratch tapes in the tape library (maxscr=9999, rather than pre-defined
volumes which cause mount request time-outs), so any file or "glob" greater
than max-size goes to the nextstg pool.

Don France
EDS Infrastructure Engineering
San Jose, CA
mailto:dfrance AT pacbell DOT net 
PACE - http://www.pacepros.com 
Bus-Ph:   (408) 257-3037