Peterm69uk

This is my set-up, I love these primes, previously owned and sold these before buying them again.

From what Ive been reading on other forums it's not just Lexar but I do have the opportunity to return the Lexar cards and I'll try the SanDisk Extreme PRO 32 GB  280MB/s cards. Seeing as Raw writes to slot 1, where I'm having the problems, the increased data may be the problem.

Just tried out the Sandisk 64GB 280 MB/s UHS-ll card, an error appeared straight away.
 
 So looks like it's firmware/hardware related 

I shot an entire event between AF-S and AF-C, both CH and CL, no freezing and no hiccups. I use SanDisk Extreme Pro cards. 

Update:
 
I have no freezes in the past week, since I speed matched the cards  in both slots: going to the 2000x cards in both slots seems to have eliminated the issue for me. Which is good, if annoying, because it seems a waste to have such a high speed card in the JPEG slot. Oh well.
 
For anyone collecting data, my freezes happened in the following AF Modes: Single Point, Single Shot and in 3x3 zone mode in Continuous Focus mode. In both cases, the drive setting was CL, which I have set to 5 FPS. Camera is permanently in boost mode.

I use 2 of these, so far no issues on my side.