2017 iMac won't read SDXC Card
I have a similar problem with my iMac 2017 and a 128 GB SD card (SanDisk Extreme Pro SDXC UHS-1).

https://discussions.apple.com/thread/250474343?answerId=250905207022#250905207022

My iMac (OS 10.14.6) refuse to mount the 128 GB card.

It's a brand new card and I use it in my digital camera. The card works as it should in my camera and I'm also able to mount the card on my Macbook Pro (2009) OS 10.11.6.

I have other SD cards (32 GB) that I can mount on my iMac, so I know that there is no hardware problem with my iMac's card reader. 

It's weird that the 128 GB card works on an older Macbook Pro which is running an older version of Mac OS. But it doesn't work on a newer model (iMac 2017) which is running the latest Mac OS version.

I think it is a software bug that Apple needs to fix. Although I have a feeling that Apple does not prioritize it. So the best thing is to use another computer than Apple and their latest OS, or to not use 128 GB cards with your Apple computer.