beagleboard-xm rev c --> ubuntu 11.04 (natty) --> USB keyboard and mouse not working.

When I boot my beagleboard-xm rev c with the shipped angstorm card,
press reset as soon as the mouse is detected and then insert the
ubuntu netbook imaged card, usb keyboard and mouse get detected. can
you please let me know why it is so?

Regards,
Venu