How likely is it that backups made against a failing hard drive will be corrupt?

Question

I had the hard disk portion of a fusion drive fail, starting with slow access and culminating in unable to boot in a period of a day. I have regular Time Machine backups. I'd much rather restore the system from a backup once the hard drive is replaced, but I'm concerned that there may be corrupt data due to backing up a failing drive.

How likely is my concern? Is there a way to check for corrupt data? Is there some age of backup (3 days before failing for instance) that would decrease chance of restoring bad data?

Answer 1

There is no way to say for sure.
Your backups could have corrupted files going back for many days.
Or everything, except the last few hours, could be OK.

Best approach would be a fresh OSX install on a new drive and then pull your data-files (documents/photos) from the backup. At least files backupped before the issue started (and that didn't change while the problem was present) will be 100% OK in the backup. Anything that is newer could potentially be affected.
For those files you will have to inspect them one by one and go back to an older backup copy (if there is one).

I would pick a date (say 1 week before you hard started to notice the problem) and manually inspect all data-files younger than that point for corruption.

If you don't want to inspect everything first, simply pull the latest copy from the backup, but make sure you keep that time-machine backup unchanged. When, while using the recovered files, you discover that one is corrupt you can always try to get an older version from that backup.
But don't use that same backup to start backing up the recovered machine. Start a completely fresh backup-set for that!

Good luck!