I had installed an OWC Data Doubler in my 2011 MBP and all was fine until the now-external SuperDrive stopped working properly. So I swapped the SuperDrive back into the MBP only it didn't work there either.
I put my HDD back in and THAT didn't work! Something was wrong. Careful inspection revealed the SATA cable had been flexed too much and sustained two cracks, severing a couple of circuits.
Having re-assembled my MBP several times with the various swaps I had done until this point I managed to do the whole operation from memory – being VERY careful with pressure on the new SATA cable. When I put it back together again, the HDD worked. The SuperDrive was still dead, so I ordered a Blu-Ray drive to replace it.
When the instructions say "fragile" they mean fragile. Take care with that SATA cable!