I work off of a Laptop and use an external hard drive for my capture scratch and all other FCP documents. Sometimes I need to work on the road and not bring along the external hard drive. So I create a new temporary "Final Cut Pro Documents" series of folders on the laptop hard drive.

Every time I do this, when I need to go back to using the external drive, FCP get very confused when I switch things in "system settings". I get duplicated folders and all sorts of problems. I also have a hard time bringing the projects I did on the laptop onto the external drive.

Is there an easy way to switch between drives with two different sets of FCP Documents?

I've seen this exact question posted before (was it you?) and it's the same answer:

You're going to cause FCP to become highly unstable as time goes on because you're forcing it to constantly re-post where it's cache and temp files live - and possibly even your project files. You could even easily cause project corruption by constantly changing up the metadata in the project file as to where all it's assets live.

In this situation you'd be best off leaving all the FCP temp/cache/render files on the internal HDD.

Assuming you're using a MacBook Pro however, if you can afford a few bucks to invest in an upgrade then I'd highly suggest getting the MCE kit which allows you to install a second HDD in the optical bay of the MBP and moves the Superdrive off to an external enclosure:

MCE OptiBay Hard Drive for MacBook Pro, MacBook, PowerBook G4, and Mac mini (http://www.mcetech.com/optibay/)

One caveat: The Apple DVD Player won't work if there's no internal DVD drive connected so you'll have to use third-party software to playback DVD's. This won't affect making DVD ISO files from DVDSP4 however as it can burn to externals no problem.

If you're not using a MacBook Pro then you'll be forced into keeping the FCP reference files on the main internal drive.