In Backup Exec 2010 I have specific jobs setup to use specific media sets. Job A runs on day 1 and does not find an existing tape for his media set A, so he allocates a new tape and writes the backup data to it. Then Job A runs again on day 2 with the day 1, media set A tape, which is nowhere near full, still in the library and the Overwrite Protection set to Infinite-Don't Allow Overwrite and Appendable Until set to Infinite - Allow Append.
Instead of using the existing tape in media set A, BE allocates another new tape, assigns it to media set A and writes to it. This doesn't happen every time, just often enough to annoy the crap out of me. My library holds 48 tapes and I have 5 media sets assigned to various jobs (some media sets have more than one job assigned to them - i.e. I have both DATA and Active Directory jobs that both use the DATA media set). Of these 5 media sets, 3 of them have two active, non-full tapes in play.
I don't understand why BE would allocate a new tape to the media set when there is already one in use, available and ready for append.