I installed the ubuntu 12.04 into my flash drive to create a bootable usb stick. The first time boot is smooth with no issue. But after played around with it and rebooted it, the ubuntu could not get start. I tried both pendrivelinux and universal-usb-installer to create the bootable usb. They both have the same issue, but giving two different errors. The previous one stops with "kernel.thread helper" error. The later one ends up with a shell command environment, but can't boot ubuntu. My flash drive is 4 GB, and I reserved 2GB room for the persistence. So I can see a 2GB casper file in the flash drive. After I delete this file, I can boot the ubuntu system again. So I guess it might be some issue with loading the persistent data. Any thought about how to fix this issue? Thank you very much!