When it's 4 GB or 6 GB, it works very well, even if a bit slow since I installed El Capitan, this is why I'm upgrading from memory. This machine is supposed to be expandable to 8 GB. Any combination of 6 GB, regardless of the RAM use what slot, also works fine. I already tried all of the various combinations of these 6 Rams (2 old 2 GB and 4 new 4 GB). They are 204-pin DDR3 SO-DIMM 1600 (PC3L 12800) Rams, which seems to be the correct type. I've tried 2 different sets of 4 GB RAM, a couple was second hand, the other Crucial nine pair. because when I try one of the new RAMs (4GB) with one of the old RAMs (2 GB), it works fine. I'm sure that's not on the Rams is not inserted properly or dusty slots etc. The Macbook I have is a mid-2009 and OSX El Capitan.
I saw that others had the same question, but mine seems to be a little different. I am trying to improve my Macbook Pro 4 GB to 8 GB memory, however, when I start the laptop it emits 3 beeps, without loading the operating system and just keep it. MacBook pro emits 3 beeps after upgrade memory