First find the disk device:

    bash$ mount
    /dev/disk0s3 on / (local, journaled)
    devfs on /dev (local)
    fdesc on /dev (union)
    <volfs> on /.vol
    automount -nsl [148] on /Network (automounted)
    automount -fstab [160] on /automount/Servers (automounted)
    automount -static [160] on /automount/static (automounted)
    /dev/disk1s1s2 on /Volumes/Xcode 1.2 (local, nodev, nosuid, read-only)
    bash$

In this case, I want to unmount the Xcode cdrom. So I run:

    bash$ diskutil unmount disk1s1s2
    Volume disk1s1s2 unmounted
    bash$

diskutil can also be used to eject disks, e.g. to to eject the unmounted disk:

    bash$ diskutil eject disk1
    Disk disk1 ejected
    bash$

-- DarrellSchiebel - 15 Jun 2005