First do not stress, Macbidouille/Hardmac is not going to turn into an Apple TV-dedicated website. We are simply reporting all information about this product available for only couple of days. And we are mostly interested by the way to add features or hack some limitations; as the Apple TV looks to be prone to be modified, as the Apple II used to be in the past.
Despite official Apple statements, the website wiki.awkwardtv.org reports that the USB port is fully functional and not reserved for maintenance, as one can boot in single user mode by plugging a USB keyboard and pressing the 'S' key when booting.
Even better, the following video demonstrates how to boot the Apple TV from an USB external HD:
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You simply need to press simultaneously the 'menu' and '-' buttons from the remote control to boot from a partition located on an USB external HD.
This trick is important, as it allows users to modify the system while giving access to larger storage space without disassembling the machine to change the HD as we published the procedure this morning.
[translation by Linathael]