One drive going down or more than one?
Is the drive going down on one media server or all media servers?
First step is to determine if one media server is having a problem or all media server and if problem is seen on specific drive(s).
Have a look at <install-path>\veritas\netbackup\db\media\errors file on all media servers to narrow down the problem. You can also copy the files to .txt files that correspond with media server names (media1.txt, media2.txt) and upload here as File attachments.
Next step is to enable bptm log folder on all media servers to troubleshoot I/O errors from NBU point of view.
To enable logging for Media Manager (hardware), add VERBOSE entry to <install-path>\veritas\volmgr\vm.conf on ALL media server followed by restart of Device Management service.
Exact reason for drive being DOWN'ed will be logged in Event Viewer Application log. Hardware errors will be logged in System log.
There is no issue with tape drive or Media.
NBU is merely reporting these errors. I/O error means that something is indeed wrong with either media or something in the data path which includes hba, hba driver, cable, switch port, tape drive, tape driver, etc...
Above steps should help with pinpointing the problem.