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.
Many storage systems have features called “data scrubbing” https://en.wikipedia.org/wiki/Data_scrubbing , which Synology discusses here: https://blog.synology.com/how-data-scrubbing-protects-against-data-corruption
This will correct errors with drives, and potentially give some early warning that a drive may fail. You will also want to run SMART tests on your drives. Quick tests often (I do daily), extended tests occasionally (I do monthly).
The backup software should also have a way to verify the accuracy of the data, and check that the data can be restored. On Synology, HyperBackup has backup integrity check https://kb.synology.com/en-us/DSM/tutorial/What_is_backup_integrity_check_for_Hyper_Backup_tasks .