> Replace your Time Capsule
We use a 3TB Time Capsule to back up our two Macs. That’s almost full, and we want to upgrade both of them to Big Sur. What’s the best replacement storage option?
To get the best from Time Machine in Big Sur it should back up to APFS, which means it couldn’t continue using your Time Capsule unless you were to delete most of your existing backups. As that’s now over three years old, its hard disk is becoming increasingly likely to fail. If you possibly can, now’s the time to put that drive as an archive of those old backups and start backing up to new external storage.
Time Machine in Big Sur will back up happily to networked storage, but is significantly slower when doing so. If one of your Macs is running most of the time and tends to do the heavy lifting, you could connect local storage to that, and make it available over your network using the Sharing pane. If both make heavier demands, then buying the best NAS you can afford could work out, provided that you can accept longer backup times.
The best scenario would be a 2TB external disk attached to each of your Macs so they can backup locally to APFS.
Connect your new storage before you start to install the Big Sur upgrade. When you’re ready, make a full copy of each Mac’s Data volume from the startup disk to the new disk(s), using a utility like Carbon Copy Cloner (bombich.com) or SuperDuper! (shirtpocket.com) in case the upgrade runs into problems.