How to set scratch Disk.
1) Start Final Cut Pro.
2) In the menu at the top click on Final Cut Pro... System Settings
3) Then select the tab for: Scratch Disk Settings
4) Near the top of the screen that appears you'll see a series of checked boxes for Video Capture, Audio Capture, Video Render and Audio Render.

Waveform Cache Set
Thumbnail Cache Set
Autosave Vault Set
These are three other Final Cut Pro settings, each of which also should be set to the same scratch disk as the first one.
It is especially important that you set the Autosave Set button to the same scratch disk, as this is where Final Cut Pro will periodically save copies of whatever you're working on so you don't lose your work if the computer crashes. You want to be sure that these automatically saved files are being saved to your scratch disk drive and folder.
Make sure all three Set buttons have been set, and follow the same directions as above to use Choose a Folder to set each of these to the same scratch disk.
When you're done and back at the Scratch Disk Settings Screen, leave the default settings for the other options, such as Minimum Allowable Free Space on Scratch Disks, Limit Capture...Size and Limit Capture Now.
And click on the button at the bottom labeled OK.