Hi Kenny Q,
I totally agree with Christian... leave logic and all other applications on your system disk and move 'other files' to external HD's.
For example I have 3 external HD's (of various capacities) one for writing logic projects to, another for photos, movies, documentation, etc... and a third that is used only for system back up.
I've been using this system for a couple of years and have not encountered any problems.
Plus it helps keep 'things' organised and easy to find.
Hope this helps... :)
Cheers, Bj