Answer to "Can we install a second internal disk drive?"Yes! If you have a SSD model you can add a second internal drive, the 2012 SATA cable sold by iFixit works just fine. You can see my upgrade at grayunicorn.com.
Answer to "My headphone jack sometimes not responding to headphones."@night4cat, exact same problem here - Headphones not detected when plugged in after sleep, same thing when the port is used for line-out or with another headphones set. This is almost certainly a firmware issue since the machine never fails to recognize the headphones after a reboot. It is not physical port damage. For now the best workaround is to use a USB DAC such as the USB adapter supplied with Beyerdynamic MMX2 headphones, or just reboot. For a real fix I think we will be waiting on Apple for a software update. Update: Hey night4cat: if you don't know how to measure impedance I strongly recommend you don't do it. Firstly, it's pointless - this is nothing to do with hardware. Secondly, you have a warranty and you should be taking your machine to an Apple authorized service centre if you still don't believe me, not tinkering with it yourself. If you do mess with it, and you create a hardware problem by incorrectly applying meter probes or by some other method, then you will have a hardware problem and...
Answer to "Opening keyboard WITHOUT ruining it"Model A1314 (plastic antenna cover located centrally) can be disassembled. Under the plastic cover there is one screw, this frees the switch/pcb/battery assembly. When the keyboard ribbon is disconnected the whole thing will slide out sideways. The white plastic base can be removed carefully by getting a couple of knives (use a sharp but heavy blade, fixed or locking, never a folder) under the short edge. Then carefully pull it off without bending or yanking, the adhesive will let go with no added heat. The keyboard plate is fixed to the aluminium chassis with many small welds which I did not try to break. There must be at least 40 or so. I did this to a broken keyboard that had a hot tea with milk spill over it, all the keys were gummed up. After removing everything I could it went in the dishwasher and the keys now operate freely. In a couple of weeks when it has had plenty of time to dry completely I will reassemble and try it out. It may not recover, but the point is you can disassemble without...