10.12.3 crashes FCPX

After updating 10.12.3 FCPX seems to be acting up and not stable.
I am guessing there is a issue using RED RAW footage (which is FCPX biggest benefit) .
FCPX will open up but as soon you start editing or want to playback a project the beach ball of death will appear and hang.

Now this is where it gets wild! Everything worked fine in the recent Library before updating . Having experiencing the issues above on my Mac Pro I decoded to move the Library (RAID Pegasus) over to my iMac. I get the same effect but FCPX and macOS are not up to date with the latest version they are both one version behind.
If I create a new library and copy some of the clips from the Library I worked on, it also repsonce the same. Especially when applying the Flow Disolve.

So it is clearly not the Library must be something.... could it be the footage somehow got corrupted?

Well, that's not good. Does it crash and produce the crash dialog or just indefinitely hang? If it actually crashes, could you post the stack trace of the crashing thread?

Is it limited to RED RAW or does it happen with any other codecs?

It's odd it happens on the previous versions of FCPX and macOS, and when using a test library. What if you re-import the clip it's failing on and try that in a test library? I wonder if there is some conflict between 10.12.3 and the Red Apple Workflow Installer, which is still on 11.0.0 dated 10/11/16.

Re corruption of media files, normally you wouldn't expect that since the editor should never write to the file, or even open it in write mode. However anything is possible and given your symptoms it's a logical question. if you restore a backup copy of one to a separate folder you could do a binary file comparison. The console tools diff and cmp should work, and there are a bunch of graphical file comparison tools listed in this thread: apple.stackexchange.com/questions/10099/...can-i-use-under-os-x

My suggestion would be to delete all of your render files. This solved a similar problem for me.