I have a similar problem with my iMac 2017 and a 128 GB SD card (SanDisk Extreme Pro SDXC UHS-1).
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 ...
"Apple doesn’t recommend storing iMovie libraries on external storage devices like SD cards and USB flash drives, or drives shared on a network."
'cause the quality of external flash cards are poor. I have like eight flash drives bought in the last eight years and two of them have failed. External cards use a double layer of circuits where as internal ...
An iMovie library holds all the media and data used to make movies in iMovie. If your iMovie library is large and you want to free up storage space on your Mac, you can move your iMovie library to an external drive.
I've never heard of any reason against storing photos, movies, and music on external volumes, except for confusing the applications if the drive gets removed while in use, or if it is not there at launch.
I use an external USB drive for my media, as my internal storage is not expensive large enough, and have not had any problems.
You'll need to check ...
I suspect the encrypted vs unencrypted read and write speed has to do with continuous vs random performance on (Micro-) SD cards as measured and described here: https://www.jeffgeerling.com/blog/2019/raspberry-pi-microsd-card-performance-comparison-2019