Eu pesquisei um pouco no Google e encontrei essa resposta no tópico Compartilhando drive USB externo pela rede nos fóruns macosxhints:
You could also use NFS. i'm not in front of a mac system right now, but it would go something like on the mini using terminal
$share /Volumes/<usb_drive_name>
and then on the powerbook
$mount <computer_ip>:/Volumes/<usb_drive_name> /Users/<username>/Desktop/<mount_point>
You would obviously need to fill in your information where you see <> above. for the mount_point, just create an empty folder on your desktop and substitute the name of the new folder for .
Of course, this is a very UNIX-y way of doing this. it could be that you are just looking in the wrong place for your usb drive. you can't just share out your desktop and find mounted devices there. you need to share /Volumes because that is where periferals are actually mounted. like i said, i'm not in front of a mac right now, so i can't check.