So I'm upgrading the memory in a mid-2009 13" MacBook Pro 5,5 2.53 GHZ laptop. It came with 2 x 2GB DDR3 PC3-8500. Based on the specs I found here: http://www.everymac.com/systems/app...3-mid-2009-sd-firewire-800-unibody-specs.html I purchased this Corsair 2 x 4GB upgrade kit, but when I try to boot up with either one or both of the 4GB chips I get the slow beep memory error. I decided to try the chips in an Acer laptop just to see if they worked, and they did. And while I was at it I took the 2 4GB chips from the Acer laptop, which are actually these PC3-10600 chips. So the 1066 PC3-8500 chips the machine is (I believe) specc'ed for do NOT work but the 1333 MHZ PC3-10600 chips do. And based on the fact that the 8500's will work in another laptop (albeit not a Mac) there's not any immediate indication that they're defective. Also, since the MBP won't boot with either one of them they would BOTH have to be defective for that to be the explanation. So . . . any ideas? Am I going crazy?