[dnanian]

I moved the thread for you.

The question here is if you've made a Sandbox. Naming the volume "Sandbox" doesn't mean it's a Sandbox. If you created it with the "Sandbox" scripts, it uses the original drive, too. That's the point of the Sandbox, and is what I've said above.

If it's not a Sandbox, and is, instead, a backup created with "Backup - all files", then if you name it the same as the original drive when you start from it, you should be able to unmount the other drive. If it's a Sandbox, though, you cannot unmount the original drive because it's in use, due to the shared files.