Unable to boot chromebook from USB

[James Yi]

I'm trying to boot my chromebook from usb, I has installed mr.chromebox script, but using that script I will need to repair original chrome os every time I finish usb boot. So I decided to use the original chromeos developer mode usb boot (the ctrl-u one). I seted dev_boot_usb to 1 and rechecked it as "dev_boot_usb = 1 # [RW/int] Enable developer mode boot from external disk (USB/SD)". But I still can't usb boot with ctrl-u, every time I press ctrl-u it will just beep and black screen for hald a second.