What happens if you type M:\ into File Explorer's path field and push return? Does it show the (empty) drive then, or an error message?
If you open Disk Management (type "partition" into the start menu and it's usually the first match), does it show anything about that drive there?
Make sure Opus isn't running Elevated or "run as" as a different user account to normal. (See Why NOT to run Opus as Administrator under UAC for more detail.) That can result in different drive letters to software run normally.