I think someone should probably "tart up" my prose, correct any issues with my experiences and raise this as a sticky, as some people will otherwise run into problems with this receiver.
I have had the ET8500 since 1st shipment in Germany (January 2015), and is my all time favourite, although flashing/booting needs more attention than some receivers as the bootloader supports multiple images (OS1 and OS2). (I am assuming that all ET8500's shipped from WoS come with the multiboot bootloader (it should ).
USB Flashing.
I alway remove everything except the usb stick (with the ET8500 image folder) inserted in the side USB port (my preference - see later)
Power up and the red light (on bottom left of LCD) will change to flashing blue light if the USB stick is detected - touch the blue flashing button - LCD display should show 3 lines - at top "Install" then OS1 or OS2 - with the remote (QUICKLY) choose where you want to install (OS1 or OS2) by moving the cursor up or down and then pressing enter on the remote........ Wait for the ET8500 to reboot.
1) If you do not choose it will go with the current selected image - not always what you want if you have a working image and you want to try/test the next image update.
2) Once rebooted, you will go through the normal wizard setup - the LCD is far faster than the TV picture, so I always setup the new image via the LCD.
3) Once the flash has started I always insert my USB stick containing the settings backup's in the rear port so that I can restore everything immediately from the wizard.
I find the multiboot particularly useful, as long as you backup your settings from a working image (say OS1), you can then install the next image on OS2, apply settings and you can test the new image, giving you a way of "rotating" your images safely.
Couch Image flash
Works as per normal, except (and I haven't tested this for a while ), it will only flash image OS1. Applying a couch flash on image OS2 on my boxes has in the past always flashed OS1 - but circumstances may have changed.
Booting
On a reboot/power up the LCD display will indicate "Boot" and give you a chance of selecting OS1 or OS2 with the remote, or if no response the last used.
Enjoy