The md does not have a password for root.
If you need a root shell type sudo -s
If you need to login to an MD before it has been added fully by the system... as in the case where you are trying to add new display drivers and therefore have not run the AVwizrad and finished the Setup wizard on the MD in question you can do the following;
chroot /usr/pluto/diskless/NN passwd
(**where NN is the device number of the MD or 'Moon' in question**)
After that you can use the following line to login to the MD;
(**again NN is the moon number for the MD in question**)
Hope that helps