I did not find any solution for my problem, so I'm asking this here:
My 2017 iMac won't read a 128 GByte SDXC Card (used to store photos in my Canon 5D Mk IV). It mounts and (most of the time) the files show up in Finder. But if I try to copy or import any of the photos from the card (for example into Lightroom), my mac tells me that it can't read the file because it's corrupt or something. I tried several solutions from this and other forums, but none fixed the problem.
The Filesystem is ExFAT and it's a Lexar (95 MByte/s) SDXC Card. My iMac has no problems to read from a 64 GByte SD Card from Samsung.
My PC and Windows Laptop can copy files from both cards without any problems (so it cannot be corrupt files or a corrupt filesystem).
Kind regards, Frank