I don't know exactly whats up but my 27" 3.1ghz i5, did the 3 beeps, and I identified that the bottom left slot was the issue. It came with 2x 4g mods, one in bottom rt. and one in bottom left. When i took the left one out, its working on 4gb with the solo module in the right. Ok, so i ordered a new 4g module from OWC, which came with 4 boxes (I'm not tech), unlike the originals with 8 little boxes on the module but it is a 4gb. I put the new one in and it still beeps. So I took it out and put it in the right, solo. It worked. I suspected there was something wrong with the left slot itself (?). I then put the original module that was in the left, in the upper slot above the one on the right. (it has 4 slots). Bingo. Running at 8gb RAM again. I then put the new one in the top left slot this time, and 3 beeps. From here I stopped, since I have my desired 8 gb RAM back. What should i deduce from this ? thanks for any help.