AN update: I've moved everything I could from my C: drive to my external- cept for my games and main programs (Photoshop, SAI..etc). My C: drive has space for 224 GB, have only 40GB free. I've done both a disk clean and disk defragment on my C: drive and there is about 100GB worth of space unaccounted for. Maybe there is a black hole in my C: drive?
Gah, I'm sorry, I know there probably is a better forum to go to, but I thought I might as well see if I could get any answers here.
black hole? no. You're probably not accounting for your OS as well as basics like antivirus, flash, drivers, system updates, and utilities. If this is a desktop and you have the space you may want to look at installing a second hard drive.
It is best for you to preserve installer and exes (files you need to start install and its content) in external while leave whatever in C. If you install them in external there is a good chance for things go wrong, and they are most likely only work for the computer you used to install these games. If you going to do an install then don't drag and drop, install is not the same as copying files.