OS X seems to have had a lingering "volume is in use" issue for quite some time. So much that recent versions now offer a "force eject" option.
I often find force-quitting finder and letting it relaunch solves the issue. Maybe 90% of the time. But I deal with external volumes A LOT. Two service drives plugged in at once for syncing along with my flash drive, puts about 28 volumes on the desktop
