Deployment Tools Command Prompt - not detecting usb stick

Deployment Tools Command Prompt - not detecting usb stick

I'm trying to build a standard image of windows 7 by following this guide:
http://technet.microsoft.com/en-us/library/ee523217(v=ws.10).aspx
However, when I get to the bit which says:
9 .Perform the following tasks to prepare the UFD to start the reference
computer:
a. Type list disk, and then press ENTER.
b. From the list of disks, identify the number of the UFD. It is important
that you identify the correct disk, as you will be formatting the disk.
Make sure the size and free space match what you expect for the UFD.
For some reason my 8GB usb stick is not being detected. It is working
perfectly from windows explorer, but from the deployment tools command
prompt, it just lists my single hdd which is 250GB. It doesn't show the
8GB usb stick, which is plugged and and working. I can even make new files
on it.
Why isn't the deployment tools command prompt detecting the usb stick?
I'm using Windows XP as a technician computer.