Time Machine keeps trying to use old backup drive
I recently bought a new external drive for my Time Machine backup. I moved the backup data over with no problem, and everything is working fine on the new drive. The problem comes when I try to use the old drive for anything else. The computer keeps trying to backup to the old drive, despite having a different name and being explicitly disabled in Time Machine. I have even changed the UUID on the old drive, in hopes that Time Machine would not recognize it any more, but that didn't work.
So my question is this: How can I keep using the old backup drive as a generic external drive without Time Machine continuing to try to back up to it?