I noticed that when I migrated from backing up over USB to SMB that Time Machine started using sparse bundles for backup. I was unsuccessful in using this bundle as a backup drive when connecting to the hard drive directly via USB.
I tried mounting the bundle, but macOS complained about insufficient permissions. I then took ownership of the bundle and successfully mounted it manually — still not selectable nor listed in the Time Machine settings.
Is it possible to back up to a local
sparsebundle? I would like to default to SMB backups, but have the choice to use USB3 for speed.