Why anybody would delete a file while they were verifying it Well, that was just an example; there are countless other scenarios that might crop up. For example, a stuff on a shared network drive might be deleted or moved by other users, or the network itself might fail for some reason. A user might forget that there's a file hashing operation going on and unplug a USB drive containing the files halfway through. Etc. There are just too many possibilities of stuff that might go wrong, and that's what "unreadable" is.