Problem with Dell XPS 8500 after installing MSATA as OS drive

My DELL XPS 8500 came with a 2TB drive, in which I had 2 partitions (OS and data).

I recently bought an MSATA drive and I did the following:

1. Unplugged the HDD and installed the MSATA

2. Used the DELL Windows 8 DVD to install Windows on the MSATA

3. Once the installation was complete, I re-attached the HDD

At this point, when I press F12 I can see 2 entries, both saying "Windows boot manager". One is the HDD and the other is the MSATA. The plan is to gradually move all the programs to the MSATA and turn the HDD into a data only drive.

I could boot into either of the 2 drives for a few hours, but then, suddenly, the PC turned off, then after a few seconds it turned back on but the screen was dark. I had to turn off the PC from the power switch. From that point onwards, I kept getting into the recovery menu and even when I managed to log into Windows, the same thing would happen.

Currently, I detached the HDD again and I am running on the MSATA and so far so good.

Is there anything profoundly wrong with my approach above that could cause the PC to turn off?

Is it a power issue? I wouldn't think that adding the MSATA has suddenly brought the power supply to its knees. Or is it the fact that there are 2 drives that both have an OS (again, this is temporary. I will turn the HDD into a data drive only once I install all the apps).

Many thanks,

Aris