The above video describes how to put the AppleTV into recovery mode, where it will boot from an attached USB drive. It’s a matter of attaching a usb drive, holding down the menu and minus buttons to enable recovery mode and rebooting.
The ultimate goal is to be able to get to a point where people can hack their AppleTVs without needing to open the case. To do this, work is being done to prepare a suitable recovery image for the USB drive that will enable SSH on the internal disk –Link.
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