I must admit that I had the same thing too.
My UDB HDD was originally plugged in to the top USB port but after installed Rev13, had to move it to the bottom USB port.
The strange thing was that it had always been plugged in to the top one and if I unplugged then plugged it back in to the top one, it worked (until I rebooted the Wii).
Moving it to the bottom one has resolved the issue completly.