I would check the continuity between the 550 motor leads and the frame,
as well as between the power distributions board lugs and the frame. If anywhere is bellow 1M resistance you have a problem. It certainly sounds like a case short similar to the 775 motors from last year which killed a number of cRio's (my teams included)
The fuse on the cRio circuit board is a large (1208?) white or yellow surface mount part right by the power connector on the front edge of the cRio, it should have a 2A, 5A, or 10A stamped on the top (i don't remember the value). Given the description it is almost certainly the issue. I would not recommend replacing it yourself, contact NI and they will send you one immediately, they are very nice people to work with.
there is some good information in this post:
http://www.chiefdelphi.com/forums/sh...threadid=98274