If you're not intimidated by or otherwise averse to /Applications/Utilities/Terminal, try this:
- Launch Terminal
- Enter "sudo lsof | grep" at the prompt. (Use copy & paste, but without the quotes.)
- Leave a space after the p, then drag the icon of the volume in question to the Terminal window. (The volume's path will be automatically pasted in for you.)
- Click "Return"
Note that Terminal will ask for your password, but WILL NOT display anything, not even dots, as you enter it; that's SOP. (
More: Before Terminal allows you to enter your password it will pop up an "Are you sure you want to do this?" warning if you've never used sudo before; just click past it.)
Terminal's output should tell you what's in use on your drive.
