Artifacts (green frame and blips) when exporting HD footage to QT or H264

I am editing HD 1920 x 1080 footage. Goal is to export it to YouTube for review and then to client for website.

(Have just gone through a mess of having received files at 960x540 dimensions, working with it and then having to start from scratch....so am behind for client....)

PROBLEM - EXPORTING

1.) When I export to QT (Command E)... the 6 min file is too big to upload to Youtube (2.1GB). What is best compression to use to maintain quality? I tried doing a QT exports, then use a MPEG Streamclip, but the file came out larger at 3GB. And going through the FCP Quicktime Conversion takes an hour.

2.) When I export to QT (Command E)... various artifacts appear: green frame, garbled video. I cannot see any issues when viewng in Final Cut, even looking frame by frame.

SEQ SETTINGS: 1920x1080 - Aspect Ratio HDTV 1080i(16:9), Pixel Square, Field Dom Upper, Frames 29.97, QT Video Settings Compressor H.264

Editing on my MacBook Pro. 75GB free space. Files are on a 1.5TB Seagate Portable Drive connected by Firewire.... - could that be issue?

FCP 7.0.3