I have a fairly old version of Daemon Tools Lite.
I mounted a 800MB ISO (Office 2013 64-bit) as a SCSI drive (from my MSDN subscriptions).
Then I find out I can't install 64-bit if I have a previous Office 32-bit. So I try to unmount but it hangs. I try to uninstall Daemon Tools, and it that triggers unmount and it still hangs.