Apple started shifting Macs from its old HFS+ filesystem to its more sophisticated APFS format with macOS 10.13 High Sierra. In that release, all SSD-based Macs were upgraded to APFS. Fusion Drives followed with macOS 10.14 Mojave. But it took a full three releases from the first APFS appearance until Time Machine backups could be written to drives formatted with APFS: macOS 11.0 Big Sur is the first release to allow that.
But you format a Time Machine drive to APFS, there are a few things to consider.
There’s no way to convert an HFS+ Time Machine volume to one that uses APFS without erasing it. Apple created an exceedingly wacky way to encompass Time Machine’s snapshot format inside the APFS format, almost as if the company hadn’t developed both APFS and Time Machine. (Insert thinking face emoji here, for sure.)
Your old HFS+ based Time Machine volumes remain valid and readable in Big Sur. You can set up a drive from scratch with HFS+ to create new Time Machine volumes as well. That’s not a problem. However, if you want to shift a drive from HFS+ to APFS, you have to reformat the drive, and that erases all the Time Machine backups. Because of the structural differences, you can’t just copy from HFS+ to APFS, either.
While APFS has advantages for SSD-based storage, there really aren’t any for hard disk drives, the most likely kind of drive used for large-capacity backup drives. I would set up any new Time Machine volume formatted with APFS, but not convert an old one from HFS+.
Big Sur APFS-based Time Machine backups can’t be used in Catalina or earlier releases. This might go without saying, but I know enough people with mixed-system setups who will ask. Not only must you use Big Sur to back up to an APFS-formatted Time Machine volume, you can’t even access the backups from a Mac with Catalina or an earlier macOS version installed.
The reason is slightly involved. APFS divides a disk into one or more containers (similar to partitions). Each container has one or more volumes, and each volume (starting in High Sierra) has a “role,” which defines the kind of volume it is. You can have several volumes in a container that dynamically share the space allotted to the container, which means you don’t have to allocate storage space to a given volume beforehand. (In Catalina, Apple added volume groups, which are used to hold the operating system itself in pieces, separating your data from system files, enhancing system security and integrity.)
In Big Sur, Apple added the Backup role, designed for Time Machine snapshots and incremental backups, and which is effectively unreadable in Catalina and earlier, because those releases simply don’t know how to interpret it. Nor does Apple have any reason to back-port that role type.
You can share the disk with volumes that aren’t being used for backups. Apple notes in its Big Sur guide on a page describing the kinds of disk formats supported with Time Machine that the backup requires the whole “disk.” This appears to be an error: it should read “volume,” as it’s just a single APFS volume within a container, and based on how it works, Apple means that you can’t store other kinds of data on the volume that’s marked in the Backup role and used for Time Machine.
Apple suggests, however, that if you want to store data on the same disk, you can add a volume in Disk Utility. This volume will contain regular data, and can be used independently of the backup volume.
This Mac 911 article is in response to a question submitted by Macworld reader Jonathan.
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