The cache is all,of the temporary files FCPX creates as you work on a project, including background rendered files, optical flow data, etc. you can clean out background rendered files via the option in the file menu. It is possible to just delete the cache in the finder but FCPX will have to recreate whatever it needs as you continue editing so there will be a bit of a performance hit the next time you open a project.
Final Cut Library Manager from Arctic Whiteness is a useful app for clearing out unwanted items in the cache. It lists what you can delete safely and shows how much space you will save by doing so, then it’s just a matter of checking a box next to the item and the app will clear that item from the cache. It does a lot of other really useful stuff too., so it’s well worth investing in.
Jeff Kirkland | Video Producer & Cinematographer
Hobart, Tasmania | Twitter: @jeffkirkland