mutant hd51

That I do not (yet) know.
Instead, figured out how to flash the hd51:
- format usb stick fat32.
- unzip the image file you want to directory hd51/disk.img in the root of the usb stick.
- put the usb in the (front of) the hd51
- power the hd51 on
- once powered on, press the button on the left front (only one button exists) for something like 10 .. 20 seconds, until the display states "flashing".
Flashing done!

Flashing hd51 from openpli or openatv works by
- insert the usb stick
- turn off the hd51 (cut power)
- press the button on the front
- turn on power
- wait for the word "flashing" in the display
Flashing done!