I have a Win 2003 Server, which runs a backup every week night via nt backup.
I am using 5 different tapes, one for every day but the backup is small
enough that we are doing a full backup everyday. This all in one schedule. I
created the backup schedule with the backup wizard by selecting a normal
backup, daily (Monday, Tuesday, Wednesday, Thursday, Friday) with new media.
I then edited the backup in scheduled tasks and added the /um switch so that
it doesn't matter which tape I use. I have also used the exact same procedure
with two win 2k servers with success. However, on this server every once in a
while (2-3) times in the week, I get a blank backup. The scheduled task exits
with a code of 0x0. There are no new errors/warnings in the event log, and
the backup log entry for that backup is completely empty. In the mmc for
removable devices there is no mount request. Before, there was this warning
in the event log at the same time as the backup was supposed to run:
"Windows cannot unload your classes registry file - it is still in use by
other applications or services. The file will be unloaded when it is no
longer in use." (Event Id 1524)
This was fixed by the UPHC (User Profile Hive Cleanup Service), but the
backup still fails with no errors.
Also note that it is not tape specific, because the days that are failing
this week are completly different that last week. (eg. Tuesday's tape worked
last week but not this week, Monday's tape never worked last week, but worked
this week.)
In addition, these group of events appear in the event viewer at the time
the backup is to run:

1. Event 7036 -- Removable Storage entered the running state
2. Event 7035 -- The Volume Shadow Copy Service was succesfully sent a start
3. Event 7036 -- The Volume Shadow Copy Service entered the running state
4. Event 7035 -- The Microsoft Software Shadow Copy Provider service was
successfully sent a start control
5. Event 7036 -- The Microsoft Software Shodeow Copy Provider service
entered the running state

And 42 minutes later
6. Event id 98--RSM was stopped
7. Event 7036 -- The Removable Storage service entered the stopped state
8. Event 7036 -- The Volume Shodow Copy service entered the stopped state
9. Event 7036 -- The Microsoft Software Shadow Copy Provider entered the
stopped state