UNetbootin failed to unmount /cdrom

I used UNetBootin to install Karmic (Ubuntu 9.10) to my laptop, but I kept running into the error "Failed to unmount partitions"

The installer needs to commit changes to partition tables, but cannot do so becaue partitions on the following mount points could not be unmounted:

/cdrom

Please close any applications using these mount points

Would you like the installer to try to unmount these partitions again?"

This is probably a result of me choosing the "install from Hard Drive" option in UNetbootin, and so I was about to give up, when I tried just unmounting ;)
Of course, ubuntu complained a bit, but when I used strong words (flags) with it, it complied. The command I used was

sudo umount -l -r -f /cdrom

which worked like a charm, and allowed installation to continue normally. NOTE: you can't use this method to install ubuntu to the partition from which you ran unetbootin...that just doesn't work.