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Hi all - I foolishly ( temporarily) dumped my old Time machine backups onto my main drive, did a fresh backup, then trashed the old ones, in readiness for installing Mountain Lion on the main drive. The installer now refuses to use that drive as it is ' used for backups'. I've tried simple things like restart etc, and checked there's nothing left on the main drive that I can see, but it won't be persuaded. Any thoughts? 

(Mac Pro with 4 drives,presently under Lion.)