Although your backups should be the most reliable thing on your Mac, they don’t always turn out to be. Whether you’re still backing up to a traditional HFS+ volume, or in Big Sur and later to a new APFS volume, knowing what to do if your backups go awry is important.

Backups on HFS+ are inherently unreliable, as the file system is well-known for developing minor errors which can grow into more serious problems. Most of us are quite used to periodically checking HFS+ backups, and making minor repairs, and this article explains how you can go about that.

Backups on APFS are more recent innovations, so less well-explored territory. Apple’s new file system is inherently more reliable, but can still develop problems. Because Time Machine to APFS (TMA) relies on snapshots, there’s another problem, as those snapshots are strictly read-only, and you can’t delete files from them. If you want to reduce the size of existing backups, the only thing you can do is delete entire snapshots, and that comes with its own risks, that it could affect the integrity of the whole backup set. At the same time, unlike backups on HFS+, there’s currently no way to copy backups on APFS.

At first sight, backing up to APFS might not be such a bright idea after all.

Using APFS to your advantage

HFS+ and APFS volumes are very different.

An HFS+ volume is of fixed size, and can only be subdivided into folders, which are all within the same file system. If you’ve got a problem with an HFS+ backup volume, it means the file system for all those backups is in trouble, and the last thing you want to try is messing with it. If you want to start a new backup series, then you’ll need to create a new backup volume with its fresh file system, and allocate free space to it.

An APFS volume uses whatever space it requires from within a container, and shares the container’s free space with any other volume in the same container. If one backup volume has a problem, just create another. This is easily done in Disk Utility by selecting the container already containing the backup volume, and clicking the + tool above Volume in its toolbar.

Once you’ve got a new APFS backup volume, open the Time Machine pane and set it to be the backup storage, instead of the existing APFS volume.

Time Machine will then make its first full backup, which leaves your backup disk with two such complete backups for the moment. But your Mac is now backing up to a reliable destination again. This gives you time to decide what to do with the old backup. Do you want to try surgery on it, removing snapshots, to reduce its size, or extract what you can from it and delete the volume?

Recovery

While you can’t make a direct copy of the first backup volume, you can still copy off as much of its contents as you wish, including apps which you may have since deleted, and old versions of documents. If you’ve got the space, you could copy off individual backup snapshots as APFS volumes to another disk. While you’re working out what to do, Time Machine is still making backups, to its fresh volume.

Summary

If you have a problem with your current APFS backup, and sufficient free space on that disk for Time Machine to make a fresh full backup:

• In Disk Utility, add a new volume to the container containing the existing backup volume, in APFS Case-sensitive format.
• In the Time Machine pane, click Select Disk… and select that new backup volume. When you’re prompted to decide whether to back up to both, choose to back up only to the new volume.
• Decide what to do with your previous backup set, which could involve recovering files or whole volumes from it, as you wish.
• When you’ve finished using your original backup volume, delete it to free space for the new volume to use.