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I bought some new RAM and installed it on my motherboard. But, the BIOS would not post. There's an LED on my motherboard that shows error codes, and it showed the error: No usable memory detected.

So, I removed the new memory and reinstalled the old memory, thus restoring the computer back to its original configuration. But, the BIOS still would not post, still giving the error: No usable memory detected.

I've ensured that the memory and power headers are seated properly. I've tried all possible combinations of memory slots, and I've also reset the CMOS, but the error remains the same. The computer was working fine before I tried upgrading the memory, and I originally assembled the computer myself.

What are some possible causes of this problem?

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You need to give us more details. What combinations of RAM did you try? What motherboard do you have? Can you get it to work with even a single DIMM? – Breakthrough Dec 2 '12 at 14:42
It won't work even with a single DIMM. I have tried all of the permutations. The main problem is not so much that the new RAM won't work, but that the original RAM won't work either, leaving the computer in an unusable state. – root Dec 2 '12 at 15:06

Since you find the same error now for the old RAM too I can think of only two possible reasons at this time,

  1. You are not seating the RAM in the right slots
    Since you assembled it in the first place I assume that you know which slots of the possible 4 to use;
    so this is less likely but should be confirmed.
  2. While working with the RAM you somehow damaged the slot.
    Try to clean the slots and the ram connectors once maybe there is dust or oxidization in the slots
    (such things start troubling once you move the ram out and don't show up while it is in place).
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