Hi,
Check the "Media Set" properties- The "Overwrite Protection" and "Append" period.
Also check the option in job properties "When this job begins".
The tape present in the tape drive should correspond to the option selected under "When this job begins".
One more thing- If the job is spanning-i.e- the job needs more than one tape-then the first tape after being fully written to, will be ejected and it will ask for an "overwritable tape" irrespective of the option selected under "When this job begins".-this is by design.
Following are the documents which will be helpful in understanding Overwrite Protection" and "Append" period:
An explanation of the "Overwrite Protection Period" and the "Append Period"
http://support.veritas.com/docs/237374
The basics of Advanced Device and Media Management for Backup Exec for Windows Servers
http://support.veritas.com/docs/192265
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