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bump! would love to know this as well

as far as the Authentication failure goes.... Root is the username it automatically uses. the password is the one that would be setup during the install process... not sure that THAT helps...since it...

I updated my Nvidia drivers and it fixed the same issue that the poster had.

apt-get update
apt-get dist-upgrade
apt-get install -y linux-headers-$(uname -r)
apt-get install nvidia-kernel-dkms

Have you tried updating the MBR? if you cannot change the bootable flag on the USB (like you can on a disk manager on windows) then rEFit wont recognize it.