RN3130 no boot menu

You seem to have skipped something here.  You went from a working system with one set of drives but a warning message, to a working unit with drives from another ReadyNAS (with no indication of whether or not you got the same error) to a non-working system.  I suppose it could be from your third step (which was a really bad idea, as it would have nothing to do with that reported error and can create new problems), but you've not given enough detail as to what you did, and thus what the consequence might have been.  It may be that the problem progressed to a point that the motherboard doesn't work, but I think that's a premature conclusion.

First of all, there is no way the voltage is actually at 18.35V, which you also confirmed.  It would fry your drives and probably more.  So either the hardware that monitors that voltage has gone bad (which would be an inconvenience, but not a catastrophe) or the configuration file that sets the scaling for the monitor has been corrupted.  Unfortunately, you now have to try to get your system back up and running before we can help you determine which it is.

Netgear does not sell parts and, unlike some earlier rack-mount models, the OS6 native systems like the 3100 aren't typically just OEMed from another vendor from whom parts are available (though I do not know specifically about the 3100).