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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 Time Machine container 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: Apple really means that the disk can only have a single container, which occupies the entire disk. The Time Machine backup, however, takes place to a single volume in that container.

You can’t access the Time Machine volume directly through the Finder and store other kinds of data on it, but Apple states you can add a volume in the same container. This volume will contain regular data, and can be used independently of the volume assigned the Backup role.

This Mac 911 article is in response to a question submitted by Macworld reader Jonathan.

We’ve compiled a list of the questions we get asked most frequently along with answers and links to columns: read our super FAQ to see if your question is covered. If not, we’re always looking for new problems to solve! Email yours to mac911@macworld.comincluding screen captures as appropriate, and whether you want your full name used. Not every question will be answered, we don’t reply to email, and we cannot provide direct troubleshooting advice.

Glenn Fleishman’s most recent books include Take Control of Untangling Connections, Take Control of Your M-Series Mac, and Take Control of Securing Your Mac. In his spare time, he writes about printing and type history. He’s a senior contributor to Macworld, where he writes Mac 911.

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