Understanding how NetBackup writes to tapes and what happens when images expire will help you better manage your tapes.
Tapes are linear - meaning that they write from the front to the back.
if we look at a tape.
BOT|day1image|day2image|blankspace|EOT
now the day1image expires on the tape - the assigned date does not change and you have this.
BOT|expiredimage|day2image|blankspace|EOT
if you try to write to this tape again it will ONLY write in the blankspace as tapes are Linear - meaning they can only append (tapes cannot write here and there like a disk can)
Now day2image expires - the assigned date of the tape goes blank - no assigned date
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Now look at it with multiplexing.
you use the tape - it gets an assigned date.
BOT|server1part1|server2part1|server1part2|server2part2|blankspace
if the backup for server1 fails or if the backup image expires then you have this
BOT|expiredimage|...