If the Storage daemon crashes during a backup Job, the catalog will not be properly updated for the Volume being used at the time of the crash. This means that the Storage daemon will have written say 20 files on the tape, but the catalog record for the Volume indicates only 19 files.
Bacula refuses to write on a tape that contains a different number of files from what is in the catalog. To correct this situation, you may run a bscan with the -m option (but without the -s option) to update only the final Media record for the Volumes read.
Do not try
mt erase; it will take a long time and is not necessary. See also the FAQ "My Catalog is Full of Test Runs, How Can I Start Over?"