How to find corrupted files and prevent them being backed up (thereby overwriting your good backup with a corrupted one)?

Question

My naive backup plan:

Mirror my files to an external drive. If my computer files, copy files back across!

But this doesn't prevent this problem:

What if a file becomes corrupted on my computer (e.g. a random photo or something), then I accidentally backup the corrupted file, thereby overwriting my nice backup! By the time I notice the file's corrupted, the corrupted version has already propagated into my backup and so I don't have a backup version to restore from.

How do you all deal with this? I'm on OS X, so a Mac/Unix solution would be nice!

Answer 1

You can't necessarily tell when a file is corrupted (or badly edited, or...); the only real solution is for the backup to keep multiple versions, so you can look through then and (hopefully) find one that's OK. Pretty much all "real" backup solutions do this, including OS X's builtin backup system, Time Machine. If you want something with more control (but more work to set up), take a look at rsnapshot.

Answer 2

There are several programs capable of incremental backup. Each backup is in a separate folder and consists of only the files that changed since the last incremental backup. So if the latest version of the file is corrupted just keep going back in time until you find a non-corrupted version. Mac's Time Machine does incremental backups.