Do not go external. USB 3 / Thunderbird external is fast for casual use, but working on movie projects? No.
Maybe E-Sata if your case has that slit.
Big SSD are becoming cheaper... I would get one at least for your working projects, and a 2T HDD SATA for storing (plus another 2TB for backups, int or ext)
"Where's Carmack when you need him?" by cadaveca