In order to boot the OF, press the left button immediately after the Rockbox bootloader pops up (a half second before may work also, it needs some timing).Dual boot now works on my YH-820!
Can you confirm that rockbox is completely runnig on your device? Or are there any issues?
I don't use the 820 so much because the 3.5mm output has a problem (Only left or right earphone is working) so maybe there are issues I didn't see.
But as far as I know, there are no issues except battery indicator
edit: another serious issue.
I don't remember it did that before but every 2-3second the mp3 player cuts the sound and then get it back 1s later when LCD on. So it's a pain to listen to music. When LCD off there seems to be no problem