ReadyNAS RN628X stuck booting after confirming reboot to install firmware 6.10.0

Yes the status remains Booting... Pressing the reset button at the back does not help to come into the boot menu and the USB recovery method does not seem to work as well. Even when the USB disk is not working I would expect some kind of change on the display to the message Booting..., but nothing happens! As to the instruction of the USB recovery method: this is not very clear by the way for the RN620 series. It says in the provided link to the document:

"3. Press and hold the Backup button, then power on the ReadyNAS system and continue to hold the Backup button for up to 15 seconds, depending on your ReadyNAS model.
- If your ReadyNAS system is an RN420, RN520, or RN620 series:
After powering on the ReadyNAS system, wait for the backlight on the navigational buttons to turn off.
Instead of the Backup button, press and hold the OK button (the center of the navigational buttons) for up to 30 seconds, until the LCD confirms that the ReadyNAS is attempting to boot from the USB drive."

For me this seems sequential: first press and hold the non existing Backup button when powering on and then after powering on wait for the backlight to go off.... So I gues only the last items should be followed and that is what I did. I switched on the RN628X waited untill the backlight was off and then pressed the middle dot for a very long time while and this all with the USB recovery key inserted in the front USB port. Again nothing happened and the RN628X is only showing Booting...

One thing I did not mention is that there seems to be some activity on the hard disks (also still after 24 hours), but not a lot though.