Only two memory slots working at a time

With all RAM installed, have you tried clearing BIOS by removing the motherboard battery and pressing/holding the power button for ~30 sec before reinstalling the battery? (Is it time for a fresh battery?)

I removed the battery last night and put it back this morning. Still nothing. But I didn't know about the 30sec thing. I'm gonna test it right away.

You shouldn't mix speed and you can't mix ranks. ECC and non ECC should not be mixed.

Dual Rank and Single Rank are fine but quad rank High density ram will show up as half the amount available if it works at all.

 Quad Rank ram is not supported.

Crucial 4GB DDR3L-1600 ECC UDIMM

CT4270279

• DDR3 PC3-12800 • CL=11 • Single Ranked • Unbuffered • ECC • DDR3-1600 • 1.35V • 512Meg x 72 •
• upgrade for Dell Precision Workstation T1650 system.

And...   ...nothing. I even tried with another battery (from the other computer which is working fine.) and the only thing is I had to fix time and date.

You did not say what version you have of Windows 7 but I guess it is the 32 bit version because the 32 bit version only supports a maximum of 4 GB part of that is used by system BIOSes and graphics memory. There is nothing wrong with your memory slots if CPU-Z and the BIOS reports memory in all slots you just can't use all the installed memory. The solution is to install Windows 7 64 bit or an OS that supports more memory.

That is really strange that CPU-Z sees all memory but the BIOS sees only half.  Have you tried a bootable memory test program like Memtest86+?

I've seen ram not showing up correctly in windows based on CHIPSET Drivers and INTEL RST Drivers missing.  AKA Precision T5500 with 64 gigs of ram only showing 8 available even though bios shows all ram and there are no errors.  Try installing INTEL RST 10.1.0.1008.

https://downloadcenter.intel.com/download/20104

Description	Version	Supported OS	Download
Intel Management Engine Components Installer This package provides the driver for the Intel(R) Management Engine Components Installer installs Intel(R) Management Engine Interface Driver, Intel(R) Management & Security Status software, LMS service etc. and is supported on the Dell Optiplex, Latitude, Precision, XPS models running the following Windows operating systems: Windows 7 32/64bit, Windows 8.1 64bit, Windows 10 64bit, Windows Server 2K16, Windows Server 2K12 R2, Windows Server 2K8. Note: This driver is required to install in additional to chipset driver.More details	11.0.6.1194	Windows 7 64-bit WT64A WB64A Windows 7 32-bit

Chipset Driver
This package provides the Intel Chipset Driver and is supported on Precision M6700/M4700/T1650, OptiPlex 9010/7010/990 and Latitude E5530/E5430 that are running the following Windows Operating Systems: XP, Vista and Windows 7.More details

9.3.0.1019

Windows Vista 32-bit Windows Vista 64-bit Windows XP x64 Windows 7 32-bit Windows 7 64-bit Windows XP WT64A WT32A

The OP said "the bios see half of the memory." so the chipset drivers should have nothing to do with not seeing half the memory in the BIOS. The OP did report that at one time the BIOS saw all the memory, just not now.

Thanks for all the answers. I tried to install Intel RST today and nothing changed.

All my 4 sticks are exactly the same (hynix 2gb).

Here are the Memory Module Guidelines from the Service Manual:

To ensure optimal performance of your computer, observe the following general guidelines when configuring your

system memory:

• Memory modules of different sizes can be mixed (for example, 2 GB and 4 GB), but all populated channels must

have identical configurations.

• Memory modules must be installed beginning with the first socket.

NOTE: The memory sockets in your computer may be labelled differently depending on the hardware configuration.

For example, A1, A2 or 1,2,3.

• If the quad-rank memory modules are mixed with single or dual-rank modules, the quad-rank modules must be

installed in the sockets with the white release levers.

• If memory modules with different speeds are installed, they operate at the speed of the slowest installed

memory module(s).

The white release levers are in slots 1 & 2 according to the picture in the manual. If you have quad rank modules in slots 3 & 4 they may not show up. From the CPU, the slots are numbered 3, 1, 4, & 2. I am assuming you are not getting any errors (amber blink codes) during POST that is displayed on the power button LED.

High Density ram is not compatible.