New Hard Drive; Now Boot Problems?

[Ownaholic]

Hey everybody, just installed a 1TB Hard Drive moments ago via 6.0 Sata port.

Before I start, specs:
P8P67 Motherboard. (Regular, not Pro)
1TB Seagate Barracuda 7200 (newly installed, causing the problem)
Older 500GB Seagate Barracuda 7200.

Now, when I boot up the computer, I get the "Please insert proper boot drive" error.
I went into the boot settings in the UEFI and only my NEW hard drive, and the optical drive, are being detected.
The old one is nowhere to be found, so obviously, my computer cannot boot because it is not recognizing the old hard drive, which has my OS installed.

My motherboard only has two 6.0 Sata ports, the rest are 3.0.
When I plugged in my new one, I figured I would make use of the 6.0 Sata port left over, which is directly ABOVE where my old Hard Drive was plugged in to.

Is that the cause of the problem? Do I simply need to switch them around?
However, why isn't my old hard drive showing up; shouldn't they both show up? Especially since I didn't even touch my old hard drive whatsoever.

Do I need to move the new hard drive over to the 3.0 satas? (I'd really rather not...I need the maximum transfer speed I can get out of this)

Thanks a million to whoever can help!

*edit*
NEVERMIND! A cable was loose, hurr-durr. Sorry guys!

 

[sudden0utburst]

ownaholic is an alcoholic

 

[cellar door]

Check for loose cables, oh wait

 

[MindoverMaster]

The simplest thing isn't the most obvious.

 

[cellar door]

I'm pretty sure loose cables are fairly obvious. Unless of course you are too lazy to check. In that case yes, it's certainly not the simplest thing.