Booting from a USB Drive

[Steve Holmes]

I got a new machine which (of course) has Windows pre-installed and
I'm not ready to dump it entirely just yet so in the meantime, I built
out a complete Arch Linux 64-bit system on a portable USB external
drive.  I also used or tried to use grub to set it up to boot.  During
the original settup session, it appeared to do all this OK but I can't
get the laptop to recognize or boot from this device.  I did not
modify the master boot record on my laptop; instead, I figured on
using the boot menu on the laptop to choose the USB drive; this method
works beautifully with an ISO image I burned to a small thumbdrive but
I cannot do the same with my larger USB portable drive on which I
installed Arch Linux.  Do I have to do anything else to make this
thing bootable?