FAQ:Why can I write new labels to my tapes but can't read the old ones?

From The Open Source Backup Wiki (Amanda, MySQL Backup, BackupPC)

Check your blocksize. This behavior can occur when the tape unit is set to have a particular block size (that may not match at read time what was used to write a tape). For example, in your rc.conf file you may have "/bin/mt -f /dev/nst0 defblksize 0" and you should verify this command is working correctly. As another example, some users put "options st buffer_kbs=256" in /etc/modprobe.conf to force a 256K fixed blocksize rather than deal with variable size.