You may need to look at whether your jobs start as overwrite or append and whether the tapes you expect to be able to use are overwrite protected at teh time the job needs them and thsi may mean you media set configurations need attention.
Basically if Backup Exec is looking for a tape to use and cannot find a suitable one it will ask you to provide a different one
This is a very complicated area but a few simple points
1) Appends add data into remaining space on a tape instead of starting at the beginning of the tape
2) Appends can only use a tape in the same media set
3) if an append job cannot find a tape that is suitable, that instance of the job will switch to looking for overwriteable (and NOT be able to switch back)
4) Overwriting destroys all existing data on the tape as new data is written to the start
5) Overwrite periods start at the end of the last data write to the tape, which means the end of the job not the beginning and does mean that to set a 1 week overwrite frequency you have to set the protection to 6 (or possibly 6.5 days ) and to set a 1 month frequency you have to set the protection to 27 days (shortest month of the year being 28 days long)
6) if you append to the tape (as per point 5) the overwrite period starts again at the end of the append job, or when the tape fills up. This means the exact end of the overwrite protection for a given tape can be a moving target if append jobs also use the tape.
7) Finally when Backup Exec looks for overwriteable media it can take them from other media sets, how it decides to do this is based on rules that are provided in the admin guide.
