Can’t find your Time Machine backup after upgrading to Catalina? Look for a backupbundle
If you’ve upgraded to macOS 10.15 Catalina and want to back out and restore your Mac to a previous release of macOS, you might have a Time Machine backup you could use to wind the clock back.
However, reports say that when Catalina performs a Time Machine backup, it updates the extension on the special package used for these archives. It changes the file’s extension from sparsebundle, a disk image format that can be mounted and browsed like a physical volume, to backupbundle. This isn’t documented on Apple’s site, and it’s unclear why this change was made.
We haven’t been able to observe or test this, but some users have said simply changing the extension back to sparsebundle allowed them to mount the Time Machine backup or use it via macOS Recovery to restore a previous snapshot.