Force unmount of a device, how to?

Make sure your current working directory (run 'pwd') is not a under ~/hd. If you are currently in that directory, umount will refuse to unmount.

Command lsof will tell you what process(es) hold a file open.

You will see this behavior if you have mounted something else on a subdirectory of ~/hd. In this case, neither fuser nor lsof will show anything. If you haven't mounted anything under ~/hd ('mount' will answer this question), then I'm not sure what to check.