I upgraded my edit machine to Yosemite and made sure all the important stuff including XDcam SXS import to FCP X - Which it does fine.
I now have a card that refuses to load. The card is a 64GB pro+ card (which I normally use with an F55) formatted and shot in a PMW500. The card is full - I mean down to the light flashing rapidly full!
The second card of the shoot has about 4 mins of material, exactly the same type of card, formatted the same just less material. But loads fine.
I removed all the files I could see relating to SXS cards from Library - Filesystem and Library - extensions and then reinstalled as per Sony's advice of device driver first then the UDF driver.
Currently the card shot on an F55 in XAVC mode reads fine, but when I put a card from an PMW500 with XDCam MPEG50 material, formatted UDF mode it comes up as "This disc is not formatted" - so at least its reading it.
This by the way is using a Sonnet Thunderbolt Express card adaptor.