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.
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