For my final instructions look below for the message "HOW-TO make an external disk usable for PC and NAS - revisited"
I posted my experience in http://wl500g.info/showpost.php?p=48361&postcount=5
You can read there how I got an external usb-disk working for both the WL-700Ge AND my pc. Maybe this information needs some refinement, so I'm wondering what others can say about it.
What I did not tell is that I first had formatted the disk with a primairy FAT partition. That gave it a valid partion table. After that I let the WL-700gE do it's tricks with it, to make it a volume and to define a share on it. But I did this weeks ago and I'm not very sure about the details.
Now I have the pleasure to be able to use the disk wherever I plug it in.
Maybe you need a Knoppix DVD or a Linux RescueCD to do this trick if you use Windows.
Any comment is welcome.