Hi,
When you create a drive pool, it is meant that when a backup job is about to run, it receives a storage device to store the data. Once the backup job is targeted to one of the devices in the drive pool, it will not redirect the backup job to another device due to any situation arised after the backup starts running. However, you can target the job to a backup to disk folder and then run a duplicate backup job when you have desired time for it. This may give you the flexibility with the tape drive as when the tape is filled up, it will prompt for another tape for completion of the backup job.
We hope this will help.