I think it has to do with which usb drive gets mounted first. If it mounts the data drive, then it can't find the system to boot. I was digging around on archlinux forum and they seem to indicate to use fstab to label your root drive and media drive.
use fstab even for the / drive? i'm normally used to adding external drives with fstab, but not sure how to edit fstab for the usb stick with arch on it.