SD card for Sony FDR-AX100E 4K?
Solution found regarding Sandisk Extreme Pro SDHC-1 U3 10
The card was formatted to FAT32, I reformatted the card to exFat (unit size 128kb, though I am not sure at all about the impact of unit size, that's something I'll have to check) .

What is really "fun" is that the camera itself can only format the card to FAT32 so I need to format it using a computer.

I'll keep this up to date.

Update:
Even re-formatted:  Samsung 64 Pro+, SDXC I 10 U3 look like the camera won't recognize it as U3.