I read many posts talking about importance of having multiple copies. but the problem is, even if you have multiple copies, how do you make sure that EVERY FILE in each copy is good. For instance, imagine you want to view a photo taken a few years ago, when you checkout copy 1 of your backup, you find it already corrupted. Then you turn to copy 2/3, find this photo is good. OK you happily discard copy 1 of backup and keep 2/3. Next day you want to view another photo 2, and find that photo 2 in backup copy 2 is dead but good in copy 3, so you keep copy 3, discard copy 3. Now some day you find something is wrong in copy 3, and you no longer have any copies with everything intact.
Someone may say, when we find that some files for copy 1 are dead, we make a new copy 4 from copy 2 (or 3), but problem is, there are already dead files in this copy 2, so this new copy would not solve the issue above.
Just wonder how do you guys deal with this issue? Any idea would be appreciated.
This why you check your backups periodically and replace the bad ones with good copies. If you’re asking how you know what’s good and bad - traditionally and fundamentally, even if many people here dismiss it, the storage already has checksums, that sneaky bitrot when the storage will give you slightly altered data (instead of saying “Error”) are so small that most people would never encounter this. Now of course serious data hoarders would use checksumming file systems, will do extra checksums for any archived data, also all archiving formats or backup formats have their own checksums too, if one would use that instead of dropping the files in the regular file system.