If Opus is completely exited then it can't be what's locking the drive. (Or, at least, it can't be the only thing locking the drive. It's always possible that both Opus and some other thing is holding a lock.)
Did you fully exit Opus or just close all the windows?
You can sometimes use Process Explorer to see which program(s) is/are holding locks on a device. Open the program and then push Ctrl-F and type the drive into the box, e.g. G:\ and then push Search. I think this will only find locks on files, not folders or the drive itself, though, so it may or may not reveal what's preventing the drive from being ejected.