APFS Time Machine on Big Sur: what happens when you migrate to a new Mac?

So if you have a Big Sur Mac and you’re happily backing up to an APFS volume with Time Machine, what will happen when you migrate to a new Mac? There appears to be very little knowledge out there. What is known is that things have changed considerably compared to the HFS+ days. I realize not many have bought a new Big Sur Mac since Big Sur came out, but there must have been a couple people who tried this. Hoping to hear back here.

If we assume you don’t buy an entirely new TM disk along with your new Mac (say because you’d prefer to have immediate access to your old backups), what will happen once you tell TM on the new Mac to back up to the previous TM APFS volume?

a) It just keeps going copying only what has changed and you see the backup tree as just one continuous tree.
b) It starts a brand new backup tree thus anything you had on there before just remains in place (and thereby reduces available space on that disk)

If it’s option b), does that mean you can no longer access the old backups from the new Mac? Or, in case you can indeed access the old backups even from the new Mac, does that involve “Browse Other Backup Disks…”? It’s technically not another disk so I assume that’s not it. But how then do you on the new Mac get to files that were backed up on the old Mac?

I do not remember the specifics off the top of my head, but there are methods to ‘inherit’ a Time Machine backup from another Mac.

A google search of 'time machine inherit’ will lead you to plenty of reading material.

No. A lot has changed since the TM on HFS+ days. Inherit works totally different now (assuming it still works at all which is questionable considering snapshots) and nobody seems to know how exactly. Apple seems super hush in terms of documentation of all things APFS too which certainly isn’t helping.

I’m hoping to hear from back from somebody who’s actually done this with an APFS TM volume recently. Thank you.

I don’t know if my experience is typical or not. I did a “clean install” for Big Sur (created bootable duplicate of my hard drive with Catalina, erased hard drive, installed Big Sur on hard drive, used Migration Assistant to move settings and files from bootable duplicate to hard drive). When I plugged in my external HDD, I was not given an option to inherit any backups. I cannot access any Time Machine backups created before Big Sur, although I see them in the Finder for the HDD–so I could retrieve them via the Finder if really necessary. Note: if you erase your Time Machine backup drive “to start over” and you’re running Big Sur, it will be reformatted as APFS.

Thanks, @Frodo8491. That is troublesome. It’s not exactly what I was looking for (Big Sur to Big Sur migration), but it doesn’t bode well for that either.

OTOH you used spinning rust for TM on Catalina which likely means HFS+ and not APFS. In that case it’s really a different issue.

My external hard drives (HDDs) for Time Machine were formatted HFS+. When I erased one of them to “start fresh”, it was reformatted as APFS. Yes, performance can be slower with an HDD formatted as APFS. It takes longer for the drive to be mounted, but I can’t detect much of a performance hit when running a TM backup or doing a restore with such a reformatted drive.