Hmmmmm what Brand of drive are you talking about here and do you have the drive running all the time or do you switch the drive off?
if you switch the drive off do you do it in the correct way? Any stored data can be corrupted if you shut down the drive incorrectly.
Is it all different file extensions to the video files you refer to? !s it only the ones you backed up.? What did you do with the originals
if you were backing up or was it actually a file transfer.?