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#21
Looked up MB. There are two MB's with that number, one p8h61-m lx R2.0 and the same only an R2.0 Plus.
Your manual is located here.
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Not knowing which board you have, I used the plain R2.0 version. The basics will be the same. In the English manual on page 1-8 is your memory location.
Page 1-23 shows how to install and uninstall memory. Memory is very sensitive. You can not static shock. Read the warnings.
If we get to it, you may wish to reseat. If you are not confident in doing this don't do it. Should you get an error in memtest86 you will need to remove one stick and retest. Then switch to determine bad stick. I assume both slots are populated.