As posted by Sicilian, slightly edited.
How to USB flash TM800 HD
1. Format a USB stick in FAT32 file system.
2. Download image of your choice
3. Extract image to root of the USB stick and rename to TM800_update*
4. Power off TM-800 and connect USB stick to rear USB port.***
5. Power on using rear power switch and REPEATEDLY press the Standby button on the front of the receiver.
6. When miniroot shows stop pressing standby button.
7. When IP address is shown press MENU on the receiver. You will see the progress of the flash on the front screen. When finished the receiver will automatically reboot.
* The file type is .bin. You will only see this if your Windows settings are set to show file types.
** (Some receivers have their front USB port disconnected for the SATA adapter)