Well, it's that time again. I just completed a battery change (MPU & Backplane) on my Alpha 2 Pro V/22 Curve machine. Following the clear, I got that dreaded "Fatal Fault NVRAM Driver Mount Failure". The same thing happened two years ago on this same machine after changing the battery, but I never discovered the cause. The batteries measured 3.2 to 3.3v. I then remembered that I removed the batteries and re-inserted them, then following the clear it was ok. I tried repeating this twice, but this time nothing. I've been beating my head against the wall, then I finally solved it.
The issue is the pair of CR2477 battery holders on the MPU has a bad connection with the battery. If you remove the batteries and carefully examine the battery holder, you will see two contact springs in each holder. I believe these contact springs have weakened or yielded over time and have insufficient contact force for a reliable connection. Using tweezers, I carefully bent the tip of each contact spring outward very slightly to increase the contact force. This time it booted fine, and I made it to the setup menu.
There may be a lot of potential causes for this error, but don't overlook a simple poor battery connection. I just lost faith in this style of battery holder.