Hallo, i installed also build 7100, i seem to have the same problem.
i did try this, go to device manager, search for changed devices, if youre lucky youre pc wil find youre usb hd.
I have a pci card with 4 usb 2.0 ports and on that card i have 4 external hd's
in windows 7 build 7100 only 3 of those hd's where found after searching in device manager.
in build 7000 there where no problems with usb hd's, my bigest problem whas audio
i hope there won't be any problems like that in the retail version of windows 7