Hello, my name is Dan.
I will do my best to help you, the only stupid question is the unasked one.
Ok, that was a good move installing a terabyte drive.
One question is what speed is the drive, 5400 or 7200 RPM? Faster is better.
Quit iMovie before doing these.
One thing I can suggest right off is repairing permissions, this PDF explains it.
Then try a pram reset, it clears many faults and will help performance.
To do a PRAM reset, you need to restart your machine and hold down these 4 keys all together at the same time.
‘Option’, ‘Command (Cloverleaf or Apple)’, ‘R’ and 'P' all together at the same time.
Keep them down until you hear 4 startup chimes, this is the sound that your computer makes when it starts up, it is like a boonngg. The screen should also cycle from white to black and then back to white along with the chimes.
The sound should also get louder.
This resets all of the system buses on your computer, USB, FireWire, display, ethernet and your keyboard/mouse/trackpad.
This will correct many faults but not necessarily yours.
Do those two and try it again, then let me know.