Thats usually way beyond basic user help and deeper into the problem. If normal TFTP recovery methods fail to get the router back to a operational state, seems the router would need to be replaced. You could check the DD-WRT forums and see if anyone has seen this there. They usually go deeper into the NVRAM side of things.
This update is for others who may have similar problems.
I've found that R8500 succeeds to boot under *low* temperature (at least 5 degree Celcius). To expose it to low temperature, I put it outside the window in this winter season. In this condition, I could get CFE prompt and also connect to its web management console through http. I could perform NMRP recovery (local netgear support guy provided) and firmware update was successful. However, after a few seconds of bringing it into the room, it gets turned off very soon.
In my opinion, R8500 is the worst product ever by Netgear. It was the second R8500 I bought. First one died only 2 - 3 days after purchase. As it was purchased from Amazon (US) and local Netgear branch in Korea denied to support it, I returned it to Amazon for refund. After that, I purchased it again from local market in Korea, expecting local follow-up support. But this time, two years later, it broke down again, I could not receive any support because the warranty period was over. They said that "There is nothing we can do for you, as its warranty has expired." R8500 is high-end product, not a cheap one, and it may break down just after two years and cannot be repaired at all? How ridiculous!
