Identify connected external disks programmatically

Question

I want to write a shell script that ejects all currently-connected external drives (either USB or Firewire drives). I can use the disktuil eject <disk-path> to eject a specified drive. But I still can't figure how to find out what are the external drives. For instance, /Volumes shows internal drives, external drives, and mounted images. Is there a way to identify only external drives programmatically?