Upload & Sell: Off
do you have one TB free, and have you told PS where it is?
within Windows version of Photoshop
on Mac is top left corner Photoshop Preferences PREFERENCES/PERFORMANCE/SCRATCH DISK
This ^^^^^ is the most important thing - you do have to "tell" Photoshop where to look for all that free space on your drive. Extra RAM shouldn't hurt, either.
(like Henrik, writing from memory...)