I was becoming annoyed with time it was taking my mini to boot up or launch certain programs from the stock 5400 RPM hard drive. So I decided to add a 500GB Seagate Momentus XT hybrid drive to act as my primary boot drive, and kept the stock drive for data storage (iTunes music, movies, photos, etc).
Taking the mini apart (the first time) went pretty smoothly thanks to both the step-by-step guide and the video provided. installing the second drive into the mounting bracket was pretty easy; however, the challenge came with putting everything back together.
I had some challenges getting the stock hard drive back into the correct position so that the hard drive cable would be in the correct position to reattach to the logic board, and so the screw holes on the drive would line up with the ones on the antenna plate. Also I had issues with getting the cable from the power supply back into the right position so that it would not force the stock hard drive out of position. I had to assemble and disassemble several times to get everything back in the correct positions.
At last, it seamed that I got everything back in correctly. I was just about to finish reattaching the fan, when I noticed that there was an extra screw still in my screw tray. I realized from where it was in my tray, that it was the one that holds in the power supply. So I disassembled and reassembled my mini one more time (getting to be an expert at this) to get that last screw back where it belonged.
Finally, everything was reassembled and I was able to reboot my mini without any problems, and started cloning my old drive onto the new one.
It's been a little over a week since the install, and everything is running great. Once I had rebooted a couple of times and started using my usual apps, the new drive started cacheing them to its SSD memory, and the difference in how long it takes to boot my system and launch those apps is night and day from the original stock drive.
For getting the stock drive back into the correct position after installing the second drive, I would recommend the following. After securing the hard drive tray back into the case with your second drive installed, stand the mini up on its end with the hard drive bay at the bottom. Look down through the opening of the case and locate the mounting holes for the stand-off posts on the stock drive. Slowly lower the stock drive into place, then slide back and forth until you feel the stand-off posts drop into the outing holes. This will help keep the stock drive in the correct position while you are trying to reassemble the mini.
Second, on the power supply. There is a bit of an overhang on the power supply on the end closest to the hard drives. When sliding the power supply back into the case, be sure to pull the power cables underneath this overhang, and using your finger to keep the cables in place until the power supply is back in position. This will help keep the power cable from interfering with the stock hard drive's position, and put the cable itself in the right position to be reattached to the logic board.