In general, one should only be possible to execute an operation after he has 
verified that he is able to do it. A problem can still occur during the process 
due to whatever circumstance. But it is the duty of the initiator to check 
whether he most likely will be able to succeed. 

Here is a good NW example: the deposit of a cartridge from the CAP to a slot 
will fail if a tape is already present at the destination.

It should not be any problem to run a SCSI Mode Sense command first to verify 
whether the tape is write protected.

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