What is the "__SYNC__" file in the .fcpbundle?

So I've been doing a little house-cleaning with my FCPX Library files (removing render files, optical flow, etc...). Also, all of my media is externally located, so a typical library size is about 25-45MB for me. Nice and small.

However, I noticed that some of them would be around 15-20GB, some even higher. After showing the package contents of a FCPX Library, I couldn't find anything that would account for that high file size for the library. But in the root of the FCPX Library I was noticing a "__SYNC__" file. Selecting it showed that it was 0kb. However, when I tested it and deleted that file, my FCPX Library would lose all the extra bloat and reduce down to the 25-45MB size that I am accustomed to. After deleting that file, I opened the Library with no problem and nothing seemed to be wrong.

Does anyone know what the heck this file is, or what it is for? And is it safe to get rid of? Thanks for any help!

Hi Adam,

That's a very good question. Try this:

Create a new Library in FCP X.
Then switch to the Finder, select the new Library in the Finder and open the package. You will not see a _SYNC_ file at the root level.
Go back to FCP X and close the new Library, then switch back to Finder and open the package again: you will see a _SYNC_ file at the root level.
Open the Library again in FCPX, the _SYNC file will disappear from the Library package.

So the _SYNC file only seems to be relevant to a Library being opened or closed in FCP X. The _SYNC_ file is indeed always 0 kB, so I am really surprised that deleting this file from a Library would make such a difference with regard to the total size of the Library. Why this would make your Library smaller is a mystery to me.

- Ronny

Hello. I have a problem with read and write permissions. Can you tell me EXACTLY where the _SYNC_ file is? I can not find it.
Thank you, Emile.