The problem when upgrading is that Time Machine can’t convert your old HFS+ backups to new-style APFS backups, as that would involve not just changing file system but completely rewriting every backup in the store. APFS backups are faster, and because they can copy changed blocks rather than having to copy whole files, they are more economical in terms of space too. APFS backups are stored in individual snapshots, each with their own file system, whereas HFS+ backups are all pooled together in a single file system which is inherently prone to error, a tendency which only grows the more backups are made to that volume. The new structure of the backup volume is far more reliable, and doesn’t need routine maintenance in the way that HFS+ does. There are many advantages with Time Machine backing up to APFS. I suspect that it will, but strongly recommend that anyone backing up to HFS+ and upgrading to Big Sur or Monterey should switch to Time Machine making backups to APFS. If you’re upgrading from Catalina or earlier, or are running Big Sur but still backing up to HFS+, it’s not yet clear whether Monterey will still be able to make backups to HFS+. As Time Machine backups no longer contain any of the System volume, the first backup after upgrading from Big Sur needn’t be much larger than normal either. Although it awaits confirmation in the final release of macOS 12, all the indications from beta-releases are that you won’t need to do anything: Time Machine should continue to back up happily to your existing backup storage, creating further snapshots which you can access as normal. The simplest migration is when you’re upgrading from Big Sur to Monterey, and are already backing up to APFS. One vital consideration is how to migrate our backups: this article looks at what you need to do for existing Time Machine backups, whether stored locally on an external disk, or on your network. It’s that time of year when many of us are planning our upgrades, either to Big Sur or Monterey.
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