Damn, sorry to hear that. I have switched to using a Sandisk 128gb SD card (UHS-I) for the past 4 months and haven’t had a lockup after 6000 frames taken. With my 512gb Sandisk Cf Express I had 3 lockups in the first 4 months (about same number of frames taken over that time)I bought my camera with 1.3.1 installed on it 2 weeks ago and mine has locked up 3 times, with different cards. required removing the battery tray and turning off and on.
I am still on firmware 1.2.0 at this stage.
Perhaps you could tell us the exact model and speed rating of your Sandisk Cards?
Every time I had a lockup in the past it was when the camera was in the process of writing to the CF express card...