Q. Why did BE use tape from a media set which is not targeted by the job?
A. When BE needs a tape to write to, it will first search for it in the following media set in the sequence listed below
1) the targeted media set
2) the scratch media set
3) any other media set
This is by design and is to ensure that the job completes successfully.
This problem will not occur if the Overwrite Protection Period (OPP) of the targeted media set is set correctly. The most common mistake is not to cater for the time the job needs to complete. The OPP starts from the end of the job, not the beginning. Suppose the OPP is set to 1 day. If your daily job is set to start at 10 p.m. and it takes an hour to complete, then the tape will be write protected until 11 p.m. the next day. When the job runs again tomorrow at 10 p.m. the tape is still write-protected. If the OPP is set to, say, 22 hours, then the tape can be overwritten and BE does not need to look for tapes from other media sets.
For a fuller understanding of how OPP and AP (Append Period) works, read the following article