My aging MacBook Pro (13-inch, Mid-2009) has developed a new problem. The memory sometimes “disappears” and one of the two 4GB DIMMs stops working. The problem manifests itself when trying to wake my computer from sleep: it won’t wake up! The computer is still running. I can hear the fan running and feels warm to the touch, but the display stays black. A hard reboot always brings the computer back to life, although this is not an ideal solution.
A few weeks ago, when I tried to reboot the computer, I got the dreaded three beeps. I removed the bottom case, removed the memory, swapped their locations, and reinstalled it. That fixed the problem. Both memory DIMMs were registering but one of them was reporting only 7.75 GB of RAM. System Profile reports Slot 1 as empty, but I couldn’t tell which one was Slot 1. As far as I can tell, the slots are not labelled.
When you look in “About this Mac” and select the Memory tab, it shows that my MacBook Pro has two memory slots. One on the left, with memory installed, and one on the right, which is empty.
Which memory slot is which? On the logic board, the memory sits vertically, one on top of another, not side-by-side. After removing a few DIMMs and placing them in different memory slots, here’s what I figured out.
|Bank||About this Mac||Physical Location|
|0||Left||Closer to Bottom Case|
|1||Right||Closer to Logic Board|
When the computer is upside down, you can see both slots. One is at the bottom and closer to the logic board. That is Slot 1. Slot 0 is the one that’s closer to you and the bottom case.
This should help you concentrate your efforts in determining whether you have a bad DIMM, a bad memory slot, or the DIMM is just loose.