So you built a Chromium OS image and copied it to a USB stick, but it only boots to a blank screen? Try this advice to override the root disk:
Depending on your hardware and disk configuration, you can try sdb3, sdc3, sdd3, etc. to find the right disk. For instance, Asus EeePC 901 has two built-in SSD drives (sda and sdb) so the correct USB device is sdc3.
You can also specify the correct root disk manually when building the USB image (inside chroot):
./build_image --usb_disk /dev/sdc3