Why would 2 sticks of 2GB RAM not work together, but work individually in my Macbook? I have an older Macbook, specifically this model, and when I am upgrading the standard 1GB that it came with. I bought DDR2 667 PNY Memory - 2 X 2GB. According to those specs, my unit can manage 4GB. I called Apple just to verify and they told me the same thing.

When I put both sticks into the unit, the machine doesn't boot up.

When I take out one, and put in the old 512MB, it boots up fine and when I get into OS X it tells me I have 2.5GB of RAM (which is right).

I have tried both sticks in both slots (all the possible combinations with the two original 512MB sticks). All combinations work (i.e. 2GB A with 512MB A, 2GB A with 512MB B, 2GB B &amp; 512MB A and 2GB B &amp; 512MB B), except with the 2 x 2GB.

Why is that?

I can't remember with certainty, but I think just the 2GB stick alone (with nothing in the other slot) does NOT work.

I am running memtest now on one of the sticks, to see if there is anything wrong with the actual sticks, so will update this once I have those results (could be a few hours though).

Any thoughts on what may be causing this and how I can fix it?

P.S. The PNY sticks are brand new.
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.