HDD to SSD upgrade seems to have gone wrong

I took the plunge and attempted to swap the HDD in my mid-2011 iMac for a 1TB Samsung 850 EVO SSD. After completing the swap, when I turn the iMac on I get the Apple chime, it sits on a white screen for a little while before getting the Apple logo and the loading progress bar, but about halfway through the loading progress the screen goes black and doesn't come back.

As a side note, to add insult to injury the fan runs full blast while most of this is going on despite the fact that I installed a OWC inline thermal sensor cable as part of the upgrade.

Can anyone shed some light on what I've screwed up in the process?