I have about 2TB of personal photos/videos, I keep a copy on NAS with mirrored disks, and a copy on a 2.5' external usb drive. Yesterday I ran freefilesync to check file contents, it took 10 hours but at the end I found that bitrot has hit three of my files on the external drive (it's very easy to see once you open the image file). Fortunately I was able to recover using my NAS copies. Now this has me thinking, is there a parity program that automatically generates a parity/recovery file for each file on a drive without one, and checks the file against the parity file for each file that has one? Even a commandline program would work as I can write a batch file. Pure checksum programs like MD5/SHA1 unfortunately won't do recovery. Quickpar/par2 would generate parity files for the entire collection at once, but I don't want to regenerate the whole parity file every time I add some photos. Plus if every file has its own parity/recovery file it makes moving them around much easier. Now before you go all ZFS on me remember this is a removable drive, and I need to be able to connect it to a PC and download images from a camera.