forthex - As far as I know, USB hard drives are *not* supported.. Pen drives are supported because they can be formatted as memory cards, but I don't think USB HDD's will work. This is something you might want to search for / double check as I am not sure.
ZeonZaku - You won't have to swap out your hard drive to play XBL if you can lock your hard drive
. Make sure you buy a hard drive that can be locked, for simplicity. Then, to play Xbox Live, you can boot to the Xenium Operating System (OS), and select to boot the Xbox with the original BIOS. It will act like an unmodded Xbox, even with a modchip installed and an upgraded hard drive. To get back to your modded Xbox, you just reboot the console, and either 1) boot to Xenium OS, and select your hacked BIOS or 2) just reboot, and if it's on Quickboot mode it will automatically load your hacked BIOS.