How to set the Scratch Disk in FCP for an external hard drive

Open FCP

Under File Open a "New Project"

Under Final Cut Pro on the upper tool bar go to "System Settings"

This will open the "System Settings" dialog box

On the second line click on "Set" this will open a "Choose a Folder" box

Select your external hard drive then click on "Choose"

Click on the boxes to the left under "Video Capture" "Video Render" and "Audio Render"

Deselect the boxes to the left of the Macintosh HD (Do Not Clear this selection)

Now set the "Waveform Cache" "Thumbnail Cache" and "Autosave Vault" to your external hard drive

Click on "OK"

Be sure to set everything back to the Macintosh HD and clear your external hard drive from the list when you are finished