This question is similar to Computer won't start with RAM in certain slots, but isn't about sticking in certain specific RAM slots, but rather the number of RAM modules.
I have a Lenovo ThinkServer TS140 (Intel Xeon configuration). The system originally came with 4 GB ECC RAM, which I upgraded to 12 GB immediately after I bought the machine. However, this became too low later on in the computer's life, and so I purchased another 8 GB ECC RAM module.
My original plan was to convert the system to run on 20 GB of memory, using all three of the modules I had thus far. However, as soon as I inserted the third module, the computer refused to POST and made repeated beeping sounds through the PC speaker.
As per the service manual of the machine, RAM has to be loaded in slots 2, 4, 1, then 3 in that order. The two existing modules were loaded with the original 4 GB one in slot 2 and the later 8 GB one in slot 4. I inserted the newly acquired one into slot 1, and that caused the issue. I also tried rearranging the modules in different ways using slots 2, 4, and 1, but the computer still refused to boot and beeped at me.
In the end, I resolved the issue by removing the 4 GB module entirely and making do with 16 GB RAM.
Why is my computer refusing to boot when I try to use three RAM modules? Nowhere is it mentioned in the service manual that this configuration is unsupported.