External drives · Upgrading, transferring and backing up the drive
Why won't Transfer Drive use a drive that is already a Storage Studio drive?
Short answer: To keep your records from getting mixed up, Transfer and Backup Drive refuse a drive already set up for this Mac. If you no longer need what's on it, erase it in Disk Utility first, or pick another drive.
Transfer Drive and Backup Drive won't copy onto a drive that's already set up as a Storage Studio drive for this Mac, so two copies of your records can never get mixed up on one disk.
If you want to reuse such a drive — for example an old drive you transferred away from — first make sure you no longer need anything on it, then erase it in Disk Utility and pick it again. Or simply choose a different drive.