Why would 2 sticks of 2GB RAM not work together, but work individually in my Macbook?

Solution 1:

This is down to a limitation in some memory controllers and how they handle memory sticks with different memory characteristics (also add into the fact that you have no control over these settings in EFI as you might have in a non-EFI BIOS)

When you put both new sticks in, the memory controller is trying to set up the memory according to the specs advised by the PNY SODIMMs, however it is having problems with that configuration and fails to initialise the memory.

When you put in a stick of the older RAM, it changes the configuration to use the slower RAM's settings and the second stick of RAM is actually running at the slower speeds and timings of the old RAM and these are invariably "slower" and more conservative than those the PNY sticks can run at.

The other posters are correct, the only solution is to get different RAM and try those. I would also advise to use a more reputable brand, the cost differences are minimal and the warranties are better.

Solution 2:

It's just a problem with particular memory. Maybe its timings are different than your system expects.

You should just swap it for another DDR2-667 SODIMM. I can't find a somehow full list of mac compatible memory so just try to search if memory you are going to swap with has any positive feedback on the topic.

Solution 3:

I have seen this many times, it just happens. The chips are most likely fine in another machine but not yours.

Return the modules and request a refund/exchange.

Try another brand.

If it happens again with exchanged/different brand RAM then your machine could be faulty. If it's an out of warranty system it would be an expensive repair.

Solution 4:

http://www.crucial.com/store/advisor.aspx

If you follow that link it will bring you to Crucial which can run a client that will tell you all the compatible RAM for your Mac. Don't know if it helps though.