It would appear that you are the victim of silent corruption. Drives can introduce problems over time with filles that generally go unobserved. Then one day the file is open and isn't what it should be. This is unrecoverable. Sorry for the bad news. If you have them elsewhere in good condition you can write over the backup. I recommend optical storage for the home user as a long-term backup to hard drive backup. I also strongly recommend multiple backup copies, on multiple media types, just as insurance.