Hello,
Sart with FC errors, server errors and verify whether or not Drives are 
indeed being resquenced or not.
Does your Tape Library allow Firmware autolevel, are all Tape drivers 
autoleved ?
Are all the Tape drive have same driver and are they loaded correctly, 
does inquire and tapes cmd verify info.
Have the servers and Tape Lib rebooted after the Maintenance ?
Do you have any errors uncleared on Tape Lib ?
You might need to verify CDI parameter on the Tape Drives.
Could there be a bad port or cable ?
HTH

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