Hi guys. Used the Step by Step instructions and had a partial success / partial failure. Also posted my question to Samsung help. Here goes.

I ordered a Mac Mini from Apple and 2 Samsung 840 Pro Series 256 gb SSD's to install with the iFixit add a 2nd hard drive kit. I removed the original drive and installed the 2 Samsung 840 Pro Series 256 gb SSD's. Upon booting Apple did not recognize the second SSD drive. After taking the mini apart many times and making sure everything was connected still no dice. Would only see the primary drive, so I had one Samsung 840 Pro Series 256 gb SSD.

To further trouble shoot, I put the original 1 TB HD back in as the second drive and the Mini recognized it. So I knew that the hard drive cable in the kit worked.

I then went to my desktop and plugged in my other Samsung 840 Pro Series 256 gb SSD into another hard drive slot. This was a Windows Alienware gaming computer. Windows recognized the drive, so the SSD drive works.

Any ideas?