Tapes are are reported as "End Marker Unreadable" when appending to media with hardware encryption enabled. This issue is observed during AVVI backups or duplicate jobs. The issue may not be limited to these type of jobs only.
Existing tape media will be rejected for the append and marked as End Marker Unreadable.
This Issue is resolved in Backup Exec 2010 R3 Hotfix 191248 and Backup Exec 2012 Hotfix 189571. Refer the related article section for the link to download the hotfix
Hardware Encrypted Backup and Duplicates