I had the exact same problem with my NSLU2 running lenny when I applied the security upgrade from 2.6.26-17 to 2.6.26-19lenny1 There were no problems during the update process. I used "reboot" to reboot the NSLU2.
I now have a NSLU2 that does not complete its boot and does not seem to access the disks since there are no information added to the logs since the shutdown.
