SD-cards not showing up when inserted seems to be a surprisingly common problem on MacOS. I don’t use my card reader a lot, but every time I’m about to install a new Raspberry Pi, I do! And, I often run into this problem: I insert the SD-card, and It don’t show up.
I’ve read dozens of articles and discussions on how to get the SD-card to show up in MacOS. They mostly contains information on how to make sure you have enabled different settings in Finder and so on, and I guess they works for some users, but if the card doesn’t show up in the Disk Utility the Finder settings doesn’t matter.
But, there’s one solution that solves lots of strange problems in MacOS, and particularly problems related to storage not behaving as expected (or not showing up at all), and it is really simple: TURN OFF TIME MACHINE
I don’t now exactly why, but Time Machine seems to interfere a lot with storage on MacOS. It is a great backup solution, and I will continue to use it, but every time I have issues with storage and/or USB I solve most of them by simply temporary disable Time Machine by unchecking the automatic backup box in Time Machine settings:Follow jonlin76