Although some of us were hoping that Catalina would support making Time Machine backups to APFS volumes, it doesn’t. However, the version of Time Machine included in 10.15 doesn’t work the same as that in Mojave, and some users are reporting problems with their backups when upgrading to Catalina. In this article I try to pass on what has changed, and how you can best address problems if they arise.

Apple’s documentation hints that something has changed, but doesn’t explain what. In one Help page, Apple points out that “if you create a backup on a Mac or partition with macOS Catalina, you can only use that backup on Mac computers or partitions with macOS Catalina.”

Additionally, in this article: “If you upgraded to macOS Catalina on a Mac that uses a Time Capsule or other network storage device as the backup destination, your existing backups are also upgraded and can be used only on macOS Catalina. New backups that are created can be used only on macOS Catalina.”

Unfortunately, Apple doesn’t appear to document Time Machine for developers or advanced users, so that is all that I can find at present.

If you back up to a Time Capsule – or to another networked storage device – it appears that this change is reflected in the extension used by Time Machine backups, which change from being .sparsebundle to .backupbundle to reflect their being updated for 10.15. Once they have been changed, they can no longer be accessed from any earlier version of macOS, at least not recognised as containing a Time Machine backup. Bob kindly reports that the new .backupbundle can still be mounted using an older DiskImageMounter app in /System/Library/CoreServices, although whether you can use Time Machine to retrieve any of its contents remains unclear.

Many users have had no problems with their Time Machine backups following upgrading to Catalina. A range of problems have been reported by some users, typically that Time Machine refuses to make any new backups to an existing backup set. The best answer then is to start a new backup set, if possible.

There are good reasons for creating a new backup set for Catalina anyway. As all system files are replaced, most other files change paths, and Catalina has a complete new layout on two APFS volumes, the first backup after upgrading is likely to approach the size of a full backup anyway. Giving it a clean start also means that your existing backups shouldn’t be converted to the new format, allowing you access to them if you needed to revert to Mojave, for instance.

This is also a wise choice if you want to run your Mac as a dual-boot system, perhaps being able to boot back into Mojave for occasional use of 32-bit apps which Catalina has rendered inaccessible. Dual-boot systems don’t fit very well with Time Machine anyway, and you should avoid trying to run Time Machine backups in both versions of macOS for these reasons.

My summary recommendations are therefore:

• Before upgrading to Catalina, make one last backup (or two!), then turn Time Machine off.
• Once you have upgraded, turn Time Machine back on but using a new backup set which won’t then be accessible from macOS 10.14.x or earlier.
• If you experience problems with Time Machine backups in Catalina, turn it off, archive your old backups, then turn it back on to use a new backup set.
• If you back up to networked storage and want to retain access to your old backups from previous versions of macOS, follow a similar course to ensure that your Catalina backups go into a new backup set.
• When running a dual-boot system, only enable Time Machine backups in the system which you use for most of your work (most of the time). Disable Time Machine backups in the other version of macOS.
• If you experience problems with Time Machine or its backups in Catalina, take the trouble to contact Apple Support. Even if they don’t solve your problems, it will ensure that Apple is aware of them, and they will perhaps get addressed in the future.

If you use a third-party backup utility such as Carbon Copy Cloner or SuperDuper!, read their support information on backing up Catalina, and ensure that you’re using the latest version of the app. Mike Bombich runs an excellent page about known issues with Carbon Copy Cloner and Catalina, and Dave Nanian gives insights into how SuperDuper! copes with Catalina in his blog. Both these products are extremely well supported: if you have issues with them, first check their online support pages, and then if you still can’t resolve them, contact the developer.

I welcome your comments and experience with Time Machine in Catalina, please.

Thanks to Hans-Peter, Bob and several others who have have been kind enough to share their experiences.