We had a similar problem, not only with NT but on Novell as well.  Let's
see if I can remember the senario.

Initial we had a disk management class which was the default MC and also
had a direct to tape MC.  When we did our initial backups we pointed to the
direct to tape MC.  After the first backup, we removed the line pointing to
the tape MC so that all subsequent backups would go to disk.  The actual
data did go to disk, but it appears the directory structure data kept going
out to tape, one tape mount for each file that was to be backed up.

Our solution:

DIRMC               LAN_INCR_DEFAULT_MC

Include the DIRMC line in your DSM.OPT file pointing to your normal disk
MC.  The first time the backup is run, all the directory data will be
"rebound" to the new MC.

This took care of the problem for us.

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Jim Larson
Jim Larson
Micron Technology, Inc.
Boise, ID