Well, in fact I did say that old and new memory both act exactly the same.No he didn't, he said the 2x 4GB sticks showed up normally.
I asked him to throw in 1x 4GB and 1x 8GB per channel, 4 in total. He did, then posted a screenshot of 1x 4GB and 1x 8GB in each channel, slot 1 & 3, the other slots were empty. This means from the 8GB sticks he bought he installed one which is ion fact a 4GB module, or else the 8GB stick he installed doesn't have any usable RAM.
It can't be any other reason because of the way Intel remaps memory on dual channel boards.
When the modules are unequal the size of the smallest density is dual and the leftover from the bigger module is remapped by the board, some hybrid single channel,
I don't know exactly how it's remapped but anyway it's not dual channel.
Another point I should make is that by one incredible coincidence I had a 4GB module go bad during a platform test yesterday, and guess what...Exact same symptoms as OP.
Only I double checked and tried post with each module, surprise...surprise, out of 4x4GB sticks, no post with one of them.
12GB shows up in BIOS main page, but in SPD page it's all there.. OS reports 16GB installed, 12GB usable. Running in single channel.
The OS doesn't have any direct access, it's virtualized.
It reads a chip which describes density size, timings etc, however when it comes to address space the BIOS doesn't assign any, because there isn't any RAM to assign.
To windows it would appear the same as hardware reserved.
I could put up screenshots but tbh I can't be assed......If the OP wants to do his own investigation, that's cool with me.
When I use old memory sticks - they work if they are in the same channel, but if one is in A and another one in B - only 4gb shows up.
There is absolutely no difference in old and new memory handling by the board.
Which means either I have 2 in-a-weird-way-faulty memory kits, or memory isn't the issue. Or maybe my MB just isn't compatible with any decent memory, many years old, or new.
Asus says: it should work no problem with 4 x DIMM, Max. 32GB, DDR3 2400(O.C.)/2200(O.C.)/2133(O.C.)/2000(O.C.)/1866(O.C.)/1800(O.C.)/1600/1333 MHz Non-ECC, Un-buffered Memory Dual Channel Memory Architecture