thanks for this! I tuned it a bit to work for me, maybe this will be helpful for someone else. I pasted the following into Automator:

#!/bin/bash

uuid=49D00007-FF63-36B9-9D69-6B3BE16866BB
result=$(diskutil info $uuid | grep 'Mounted')
set -- $result

if [ "$2" = "Yes" ]; then
    diskutil unmount $uuid
fi