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$
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DarrellSchiebel - 15 Jun 2005