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BugZero found this defect 3255 days ago.
All tape write operations fail for MHVTL if the block size on drives differs from 65 KB
Veeam Backup & Replication v9 has a new algorithm of choosing the block size. Now Veeam automatically detects the highest available block size and runs jobs according to this value. Currently this option is available for all supported devices except MHVTL.
Manually change the block size in this dialog box:
Please see Data Block Size section here