Try to see, if there is someone in some other console using it anyway. If 'anybody' is cd'd on your cd's directories you cannot umount it I think.
Use in every console 'cd' to take the user to 'the home'. CD insn't anybodys's home at your computer? Then using 'su' umount it by giving 'umount dev/hdd' (assuming that your cd is at hdd as mine...) Did it work? hv