They can also append data to an existing tape. These tape libraries can also defragment tapes. For this all the data that needs to preserved is read from the t

They can also append data to an existing tape. These tape libraries can also defragment tapes. For this all the data that needs to preserved is read from the tape and written to another tape. Even for backups you have a consolidation/defragmentation task running every so often that frees up space on the tapes and only keeps the backups you specified in your retention policy.