On my new M1 MacBook Pro Max, I have discovered that my SDXC card slot is misbehaving with a card that I have no problems with on my 2015 MacBook Pro. Three other cards are working as usual. A search on the web got me to this thread on MacRumors https://forums.macrumors.com/threads/14-and-16-m1-pro-sd-card-slot-problems.2320399/