I'm curious about this as well. I'm assuming it is either using some type of checksum for each block or the total file. There is another thread that talks about inter-sync-folder moving of files and hashing checking, if Sync is already calculating a hash or checksum for file validation when transferring could it not just retain it for that move purpose as well? Just trying to go off of another users logic of not wanting the system to recompute those all over again. -Frankly I'd be alright with a paranoid option of bit level checking, even though that means twice the data is transmitted.