[CentOS] Booting from an external USB drive?

Is it possible?

Booting the installer from the CD works, finds the USB drive, and allows me to put the root filesystem on it. I put the /boot partition on the internal hard drive and installed GRUB in the boot record of that partition.

GRUB shows the menu and begins booting, but eventually fails with the error that /dev/sda5 (the external partition) does not exist.

I was hoping (perhaps in vain) that initrd would provide the necessary modules/hooks to mount the USB device. Is there some step of the installation that I'm missing, or something extra that I need to do by hand, like in the old days when LILO had to be explicitly told about SCSI cards?