I have several 2TB drives which contain largely video and which I backup to other 2TB drives, so that they should be exact copies of each other. However, the amount of disk space differs dramatically between the original and the copy. In the worst case, the original drive has 864GB free while the copy has only 724GB. I've done disk checks, emptied all the temporary files and virus checked them to death. The actually number of files and cumulative size as reported by the backup utility shows the disks to be exactly the same.
Can anyone figure out what might be going on?