I could not get this to work, same as Paul Gibson. The adapter cable did not work, and the replacement cable that iFixit sent me did not work either. At least I suppose it's the cable because the drive works flawlessly and the Mac mini is fine. The dual-drive kit has completely failed for me. I've now exchanged the original internal drive with my SSD and put the original drive in an external USB enclosure (to be used as a Time Machine volume). So far, no problems, but it's not what I wanted. :(
Another issue: the connector on the cable for the second hard disk did not properly align with the socket on the logic board, at least with my Mac mini. It reaches about 5 millimeters too far into the logic board (behind the socket), and a bit towards the right of the socket. So I had some pushing and bending to do before I could finally secure the connector onto the socket (always afraid of breaking the cable).
So, this wasn't easy, and for me, it needed two attempts (first time with a broken cable).
Be aware of this and don't get too frustrated when you do this installation.
* Step 20+21: Took a lot of jiggling and pulling and lots of force until the power supply would come loose. I was always afraid I was breaking something.
The hardest bit was re-assembling the Mini. I found it VERY hard to replace the original (factory) hard disk. Its outer two grommets should slide into two shallow holes on the inside of the casing (hard to see), but I found it nearly impossible to achieve that, because when it appeared to be in place, the screw holes on the antenna plate would not align with those on the drive. In the end, I found it easier to attach the drive to the antenna plate first, and then slide drive plus antenna plate into the casing. Still, this needed a lot of jiggling and pushing and never seemed to fit properly. In the end, I kind of gave up and just forced the whole thing in. It worked, but it didn't feel right.
… it does say so in the manual (pull them up), but mine wouldn't move when pulling them, so I tried forcing them out towards the side. I nearly destroyed one of the connectors while figuring out that up is the only way they can move. Be careful.
* Step 3: My Mac Mini needed has three (not two) T6 screws on the fan that need to be removed. Apparently, Apple slightly changed the hardware design. I edited the guide to add this. (see also benjamin's comment from January 12).
* Step 11: I found it easier to remove the harddisk after this step, instead of removing the logic board first.
* Step 14: The screw marked yellow is no longer there if you removed three screws in step 3. (see above)
* Steps 15+16: When removing the logic board using Ifixit's special tool, there is a horrible crunching noise that made me cringe, but that seems to be normal; the board is okay. No worries :)
Apparently the first hard disk connector I was sent was defective (see my comment from January 6 above). Ifixit staff was very helpful and sent me a replacement that traveled from the US to Germany in just two days. I repeated the installation with the new cable, and it works. Thanks, Ifixit!
BUT—I have to add that I found this installation to be quite demanding. This is by far not the first time I've opened sensitive electronic devices to install parts or fix something, but still, there was a lot of sweating and swearing. Unless you have good nerves, I would actually NOT recommend you attempt this. The Mac Mini is packed VERY tightly and parts and connectors are really small; you definitely need tweezers and steady hands.
The second time I attempted this installation, I was actually afraid I had bricked the Mini, but fortunately it still works.
Some comments: the (very tiny!) connectors for the fan and IR sensor must be pulled UP to remove them, not to the side. (cont'd)
This took a lot of force on my Mac mini. A lot.
This didn't go so well. I did manage to assemble everything according to the manual, but it was one !%!$ of a fight, and now the machine boots, but the SSD I installed is nowhere to be found.
One thing I noticed is that the cable supplied to connect the second drive to the mainboard, is slightly too long. I had to bend it quite a bit so that it would line up with the connector on the mainboard, otherwise the cable would extend about 5 mm into the mainboard and not line up. I'm afraid now that I might have broken some of the wired on the adapter while bending it. However, without bending it there was no way to attach the connector.
What to do?