"Time Machine must erase your existing backup history and start a new backup to correct this." Why is that?

“Has anyone seen this problem before?”

I have, and from what I have heard from reading forums over the years, it sounds like a very common problem with Time Machine: every once in awhile, it says “I need to delete your backup and start over.”

Or it says “I can’t backup because your backup device is full” (even though Time Machine is supposed to prune itself as needed).

Or something else comes up that causes Time Machine to choke.¹

Howard Oakley has cataloged a long list of issues with Time Machine, and even has some free apps to help work with Time Machine archives (unfortunately, nothing that can “repair” a “damaged” Time Machine backup, as far as I know).

In response to this I have developed the following attitude towards Time Machine:

1. Never rely on Time Machine as your only backup.

2. Use Time Machine to multiple locations (e.g. a Time Capsule or other remote Mac plus a local Mac) with the expectation that if one gets corrupted, you will still have the other one. Note that Point #1 still stands, even with Point #2

3. In addition to Time Machine, also use at least one (preferably two) of:

• Backblaze
• Carbon Copy Cloner
• SuperDuper
• Arq

¹ This is usually where someone will chime in and state that they have used Time Machine since it was first released and never had a problem with it. I do not doubt that there are some people who have had that experience, but I tend to consider them sort of like people who win the lottery: I am happy for them, but I would never expect to be one of them, and have decided to assume I never will be. YMMV.

This happens for a simple reason. Network errors get flagged as hard drive errors, making NAS destinations too fragile for use. Don’t waste your time on any NAS that says they support Time Machine unless you can get them to actually support you when you get errors like this. When you call them, they will point their finger at Apple and Apple doesn’t support Time Machine to anything but a physical drive now.

Apple built in a failure detection mechanism and when you have too many minor errors, any drive is flagged as unreliable for adding new backup intervals and you are forced to make a decision.

1. Keep the drive in read-only mode. The data there is fine for a restore, but no new backups can happen. Put it on the shelf in case you have to restore data from when it has data, but put a new drive in the rotation to do new backups.
2. Erase the drive to start a clean slate. You lose the history, but you can then see if the drive passes the tests and can store new data safely and reliably.

If you must back up to NAS for reasons, also back up to an attached drive occasionally so when the NAS fails you, you can erase it and start fresh.

I've run into this issue a few times now. Today once again. I'm fed up with it and I'm trying out a fallback system:

I often notice the failure within a few days. So now I've decided to setup a backup schedule of my timemachine folder in Synology with HyperBackup. The retention policy is set to 60 days.

Next time Timemachine shows me the finger, I will just get in DSM and restore my timemachine data to the day of the last successful backup. It should theoretically continue again.

This time I'm late, so got to reset the timemachine state.

Fingers crossed.