Apple just introduced updates to Final Cut Pro X, Compressor and Motion. Many who have installed the updates report that Final Cut Pro X is no longer working with AVCHD files created by at least some Panasonic cameras, and reports that some Red Epic files are not working either. There are some indications this impacts 1080p 30 fps files only; however, that is probably the most popular video format for AVCHD!
Read through the Apple Support Community discussion thread for more info:
I updated a Macbook Pro to OS X 10.10.3, and installed the FCPX, Compressor and Motion updates. My 1080p60 and 3840p30 (Ultra HD) clips, all shot on a Lumix GH-4 (Panasonic camera) are all working okay, so far. I have not tested 1080p30 files.