Your disk is probably formatted with APFS which wasn't fully supported until High Sierra. Time Machine itself used the HFS+ format until Catalina, only in Big Sur will Time Machine use APFS.
If the disk is APFS you need a High Sierra or new OS to mount the disk. Even having achieved that you may only be able to access the Time Machine backup manually. If the backup is a "remote" backup and is in a sparsebundle
image you can double-click to mount it, if the extension is backupbundle
change it to sparsebundle
and it will mount.
Copying out your files is easy up to Catalina. Under Big Sur your backup disc may look empty as Apple has made entires invisible. To address that use the Terminal
to move to the disc (in /Volumes
) and you will probably find you can open the hidden folder back in the Finder (using open <folder>
in Terminal
).
HTH