Oh look another Win8 hate thread...color me suprised. This thread reeks of a pebkac error.
Sebaz I'm wondering if you had a spare SATA HDD why did you have to even have the HDD with the Windows 7 installation even connected to the machine when you installed the 8.1 preview. That's what it sounds like. Did I read your post incorrectly?
The fact that it's a preview (and might have bugs still) would indicate that taking precautions like making sure a working OS installation isn't connected to the machine that you're installing a preview OS onto is a good idea.
One thing you could have done was to partition the spare drive and put whatever data you needed to have on the new OS installation onto the 2nd partition of that drive and install Win8 onto the primary partition... while having the drive with the Win 7 OS completely separate from the machine.
then if you wanted to go back shut down the machine take out the Win8 OS drive and put the Win 7 drive back into the machine.
Feel free to disregard this post if in fact you didn't install Windows 8 while the Windows 7 drive was still in the machine.