Dual hard drive kit - Added drive not working

I have bought the mid level Late 2012 Mac Mini with the quad core 2.89GHz i7.

It worked fine with the hard drive, however, when I added an SSD (Samsung 830 256GB) to the upper slot using the kit, OS X did find it the first time, but subsequent bootups it wasn't found anymore.

I then tried removing the SSD, and replacing it with an older Intel one - I could see it in EFI as a boot drive (it had Windows installed from another computer), but when I tried to restore OS X to it, it didn't work.

I also tried putting the Samsung SSD in the lower slot and the hard drive in the upper slot, no dice. It seems for me the cable is either faulty or not working very well.

Right now the only thing that works is removing the kit and using the Samsung SSD exclusively. I had wanted to have the Samsung for Windows and the hard drive for MacOS X, but it's going to be primarily Windows to replace an aging desktop PC.

Perhaps I should've gotten a Fusion drive model and break that...

Do I have a defective cable or something?

I experimented a bit more and found out I could always see one of the drives in EFI - if I had the upper drive only, I could see it. If I had both, it would always be the lower. And the upper drive was only seen in EFI - if I tried to restore OS X to it, it never found the upper driver, only the lower (if present).