create a USB-Key, install extlinux onto it. In extlinux.conf, you can provide kernel cmdline, with root=UUID=xxx-xxx-xxx-xxx In that way, you can boot your archlinux(and any other linux) from USB, and using your main HDD(partition) as the rootfs.
Note, every time you update the kernel, you must copy the updated vmliuz + initramfs.img into you USB disk. That is not a LIVE USB-Key, it just an bootable kernel + initramfs. Not a completed OS. May be help..... .
