Which CPU and what memory are you running?
First gen Core is weird about memory, see for example https://www.os2museum.com/wp/nehalem-and-4-gbit-ddr3/

Rule of thumb is that on 32nm dual core cpus (both desktop and mobile) the largest DIMMs you can use are 4GB with 2Rx8 organisation (same as 4 series LGA775 chipset, as the memory controller is roughly the same), whereas 45nm quads may or may not run 8GB 2Rx8 DIMMs, it depends on the CPU, the motherboard, how lucky you are and what is the atmospheric pressure.
Single ranked 4GB sticks WILL NOT run on a 32nm, will probably be detected as 4GB from the SPD data but will crash horribly shortly afterwards from my experience. Will probably work on a 45nm quad as a dual rank 8GB stick is just two single rank 4GBs glued toghether.

If you ask me I'd say put four cheap 4GB 1333 2Rx8 sticks in and call it a day, you'll have more than enough ram for anything you can do with such a machine as long as you're not running it as a virtualisation host.

Latest bios for the MS-7613 is 6.18 at https://ftp.hp.com/pub/softpaq/sp56001-56500/sp56189.exe

Hi Andrea,
The CPU is: "Intel(R) Core(TM) i5". I'm not sure what generation, I haven't followed this stuff in a while.
I'm pretty sure the system recognized all 16 GB before. Or at least I should have noticed that when I installed the system, a long time ago.
A few years ago I had a problem with this system. It would shut down intermittently. I fixed it by installing an heatsink on a IC on the motherboard. I don't know if the two are related.
8 GB is more than enough for what I do with it, but I was annoyed by wasting half the RAM.
The motherboard in this system is the MSI 2a9c.

EDIT:
found the memory specs:
HyperX Fury - 99U5402-462.A00LF

Can you post a picture of the motherboard and also of the memory you are trying to use?
Also, after Intel(R) Core(TM) i5 there should be a 3 digit number, such as 750 or 660 (for example), can you say what it is?

majinga wrote on 2025-05-24, 17:28:

EDIT:
found the memory specs:
HyperX Fury - 99U5402-462.A00LF

I've had a quick look and it seems to me that those are single ranked DIMMS, if so probably won't work.

To be sure look at a module with the contacts towards you, between the PCB and the heatsink you see chips on both sides or one side only?

andrea wrote on 2025-05-24, 17:41:

...
Also, after Intel(R) Core(TM) i5 there should be a 3 digit number, such as 750 or 660 (for example), can you say what it is?

750

andrea wrote on 2025-05-24, 18:01:

...
To be sure look at a module with the contacts towards you, between the PCB and the heatsink you see chips on both sides or one side only?

One side only.
I just noticed that I'm using two different revision
2x 99U5402-461.A00LF
2x 99U5402-462.A00LF
EDIT: probably it is just the color, one pair is blue and one is black

The odd thing is that the BIOS see correctly the modules, but it show the wrong total amount.

The 750 is a 45nm quad so 1Rx8 4GB modules should work, but it's motherboard dependant at this point.
As for what the BIOS sees, the total memory is what it can actually use, while for what's in what slot it's just going from the SPD data.

Try updating the bios with sp56189.exe I've linked in my OP, maybe it will help, but I'm afraid the solution will be to use dual rank 4GB DIMMs.

I'll see to update the bios.
It will be not easy for me, because I don't run windows.
I have to decide which way to go, if flash it with a programmer, or get another HDD and install a copy of windows just for this.
This is my daily use computer, and I need it, this will take a lot of time to me, so probably I will not update this topic soon.

Many thanks for the help!