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I made a guide for this, hope it helps. Disassembling Buffalo MiniStation External Hard Drive (Thunderbolt & USB 3)
Interestingly, the mid 2011 iMac 27 inch also goes to 12GB!
I originally thought I'd be upgrading to 8GB today but found 4 slots instead of 2 :-)
As long as the two front (close to the screen) slots are paired with the exact same RAM config.
Two front slots Apple 2GB chips = 4GB.
Two rear slots, Crucial Lexar 4GB chips = 8GB
Hoorah no waste and more RAM, come on Lion... I'm Ready!
Regarding the following instruction: -
Two 3.3 mm silver Phillips screws on either side of the SuperDrive.
Be aware there are two of these on the bottom (if the slot is facing you) screws close to each other, one is for the actual drive assembly itself and the other is attaching the drive to the Mac. Beware you choose the correct one.