Actually, the same thing happened (and still does) with my 721 that I got long ago off ebay (never activated, previous owner really screwed it up, required fixing traces on the mobo etc, so i fixed it, and now just use the hdd)
Anywho, on the fresh software (was like 54 or something) , it would keep rebooting etc, had to really play with it, but namely, boot it w/o the cables, do a checkswitch, then plug em in , and do it again, that seemed to do it for me atleast!