How do I redirect the "Documents" folder to a different partition?

What you want to do is change your home directory.

You can use a symbolic link.

From terminal:

ln -s /Volumes/OtherPartition/Documents Documents
ln -s /Volumes/OtherPartition/Desktop Desktop
ln -s /Volumes/OtherPartition/Downloads Downloads

WHERE

OtherPartition is the name of the partition you've moved those things to.

Note, the Documents Desktop and Downloads directories CANNOT exist in your home folder when you do this or you'll get an error (so you should have already moved them). Also, once you've done this you'll probably want to log out and log back in.

As far as I know the desktop can't be assigned elsewhere. The others can be directed with the respective programs. I always keep my documents on another drive. I do it by using the saves as command and point to that directory. Then everything after that I can use there. For Downloads you just change it in preferences in Safari. Switch it in the *Save downloaded files to:* command.