If the scratch disk used by Final Cut Pro cuts out in the middle of an export, Final Cut Pro becomes unstable: the next time you try to launch it, it will crash on startup.

Fortunately, it’s pretty easy to fix. Delete the following preference files:


~/Library/Preferences/com.apple.FinalCutPro.plist
~/Library/Preferences/Final Cut Pro User Data/Final Cut Pro 6 Preferences
~/Library/Preferences/Final Cut Pro User Data/Final Cut Pro Obj Cache
~/Library/Preferences/Final Cut Pro User Data/Final Cut Pro POA Cache

Thanks to Andy Mees for this tip.

Resources:
Apple Discussions thread on this issue

Andy also notes that you can use the Preference Manager app from Digital Rebellion to manage these preference files.
Preference Manager