When multiple sessions configured to use the same device are scheduled, queueing is possible. The active session locks the device and cartridge in the MMDB and the remaining sessions go into queueing state and wait for the device to become free. The queueing mechanism in Data Protector randomly decides which session gets the lock. This is fatal if the operator starts a critical restore session. This restore session should get priority over any existing backup session waiting, but this is not the case.
It should be possible to start a "high priority restore" that will either win the queueing (if there is any) OR pause a running backup session currently locking the device and resume it after the restore.
Restores usually have priority over backups but Data Protector does not offer an easy way to prioritize restore.