I have a 12gb memory, 8gb DDR3 HyperX 1866mhz and a 4gb DDR3 HyperX 1866mhz.. At the BIOS the two memories were detected but it only counts the 8gb one which is on the dimm slot 1. Also when I check the This PC properties the it shows that (12gb RAM, 7.94 usable) same on the Task Manager > Performance Tab. I tried removing the 4gb memory on the dimm slot 2 and when i check the this pc properties it shows (8gb RAM 7.94 usable). I tried transferring the 8gb ram to dimm slot 2 to and still the same result (8gb RAM 7.94 usable). I tried putting the 4gb ram alone on the dimm slot 1 it shows (4gb ram 3.94) usable and when i transfer to dimm slot 2 same result. So at this point I guess there's no problem on the DIMM Slots. I also tried "Windows Memory Diagnostic" both rams were detected and no problems found. But when I check the Resource Monitor there was a 4gb + memory on the Hardware Reserved is that the problem?
Specs
APU - AMD A10-7860K
Mobo - ASUS A68-HMK
RAM - 12gb (8gb DDR3 HyperX 1866mhz and a 4gb DDR3 HyperX 1866mhz)
GPU - ASUS Strix GTX 1050ti
PSU - DeepCool DA 500M 80+ Bronze