I have an old Mac Mini (running Snow Leopard) which has been runing Time Machine backups to a Western Digital My Book Studio for several years now.
I just acquired a new Mac Mini (running Mountain Lion (10.8.1) and want to continue using that same drive for Time Machine Backups.
However, Mountain Lion insists on starting over again ignoring (and not even recoginzing the existance of) the Time Machine Backups from Snow Leopard.
When mounted on the New Mac Mini, Time machine does NOT recoginize this disk as "a time machine disk" and does not offer up the "other" backup which is visible via the Finder. (Right click, browse other time machine disks)
And yes, I can physically move the disk back to the old Snow Leopard Mac Mini and it acts just as it always did.