MDBT42Q - IDE is "Unable to Retrieve Board Information"

Unlikely that it's power since all the other boards were powered the same way

It might still be worth checking the VCC voltage - since it's a small production run it might be one component isn't connected properly.

Can you clarify what mean...

The 'hard reset' - a long (~5s) press at boot should boot without loading any of the previously saved code, so the device will advertise as it came before (MDBT42...) and should work again until the next power cycle. It won't remove any of the saved code on MDBT42 afaik, but it should still allow you to connect with the IDE and issue that reset().

Probably because we had 'save on send' set to flash - execute code at boot. even after reset.

In that case it may still get loaded, regardless of the button press state. That's going to make your life really painful.

One more option is to attempt to connect using the UART pins which may still work even if the code you uploaded somehow messed up bluetooth: http://www.espruino.com/MDBT42Q#pinout - but they may not be brought out on your board.

You could also connect using an SWD programmer if you have those pins available and erase everything that way as well.

Potentially there might be a way to have a specially formed DFU upload that erased all the saved code in the device as well, but I don't have one of those available at the moment.