Move Documents folder to different drive
There are many web pages that will walk you through this process, here is a good one:
http://www.maclife.com/article/howtos/how_move_your_documents_folder

Basically you create a symbolic link to a folder on an external drive.

Here is information on how to use the console to create a symbolic link:
http://hints.macworld.com/article.php?story=2001110610290643

And here is my take on it:


Attach external drive
Copy your Documents folder from your Mac to the external drive
Rename the Documents folder on the Mac to Documents-old
Open Terminal
Type in cd /Users/Your User ID/
Then ln -s /Volumes/External Drive Name/Documents Documents
Test it out for a while to verify that it works, when you are happy with it you can delete the Documents-old folder from the Mac