So I bought some RAM recently in one of those bins on clearance because it was opened, etc. I installed it with another two sticks I already had. All four sticks are DDR4, of course, but the old ones were 2x8GB 26666, and the new ones are 2×16 GB 3200. Unraid would only show 16GB usable rather than the 48GB. Unraid did notice them, but they were unable to be used.
After some googling which didn’t get me anywhere, I decided to remove all the sticks and just use the new ones in A2 and B2. Well, now Unraid shows 32GB of RAM usable. So next, I put in the other two sticks in A1 and B1. Boom!!! Now it can use all 48GB. I assume it has to do with a mismatch in either speed or capacity.
Motherboard: Asrock B450M Pro4