RNDP600U

If I move the single disk that I used with the working unit to either of the non-working units, nothing changes.
Both units have exactly the same reaction.

Power on, ReadyNAS appears on screen
Approximately 10 seconds later, the backup light turns solid, no change to screen.
Units sit there do nothing, regardless of how long I wait. (left running over night)

I tried using the recovery USB approach, but since I can't get the reset button to allow me to choose a boot mode, it never attempts to access the USB to recover.

I even operated under the assumption that the screen possibly was not updating and reset button was working and pushed the backup button 3 and 4 times followed by a single press of reset, to see if the boot mode was working, just not updating the screen. Nothing happens.

I'm almost positive that the 6.10.10 upgrade crashed the flash drive on both units and so long as the unit remaining, it did not need the flash drive intact. Once the units rebooted, the flash was gone and it bricked the units.

Unless there is some way to rewrite the flash drive or whatever it is, I'm not sure what options are left.

Any idea where the flash drive is located?

I stumbled across a post somewhere where someone dismantled one of these units to change the Linux to CENTOS and described a m.2 SSD drive. I wonder if the giant heat-sink is hiding a small ssd drive?