Hi, completely new on here but felt i should leave a message to say i am experiencing exactly the same thing. Don’t usually use Sony ex 3’s but just did a multi camera live switch job using them. Every time I try and launch fcp 10.4 it crashes, if I have an xdcam sd card already plugged in or if I plug one in whilst fcp 10.4 is already running. I have no prior knowledge to using xdcam with fcp prior to 10.4 so I can’t be of much help there, just wanted to let you know that you are not alone. I was able to get around this issue as we always record individual camera feeds back at the switcher via hyper deck minis, but also record internally on whatever camera’s we are using. On this occasion I was very glad I did. Hope someone finds a solution.