Bought two brand new microSD cards (
https://www.amazon.com/dp/B0735NNFLQ?psc=1&ref=ppx_yo2_dt_b_product_details) just to see if those would work and both work, but are certainly a little slow to be recognized (around 10s). At least they're not straight up throwing an error. (I didn't test transfer speeds as that's less of a concern for me than just not having to dig out a card reader)
It recognized both straight out of the package and after being formatted by my camera. FWIW both the new cards that seem 'fine' are the same type as one I already own that's been pretty flakey.
Not exactly a scientific test but another data point.
As insane as it sounds, I also tried cleaning the contacts on my older cards with some rubbing alcohol, thinking that maybe the macbook's card reader is just really finicky. Shockingly, did not appear to make any difference.
Here's where I'm at with all my cards...
Brand New 64GB SDXC (Class 10) Sandisk Formatted ExFAT >
Works every time
Brand New 64GB SDXC (Class 10) Sandisk Formatted ExFAT >
Works every time
Old SDHC 8GB (Class 6) Netac Formatted MS-DOS (FAT32) >
Works every time
Old SDXC 64GB (Class 10) Sandisk Formatted ExFAT >
Works every time
Old SDXC 64GB (Class 10) Sandisk Formatted Mac OS Extended (Journaled) >
Flakey (recognized about half the time?)
Old SDHC 32GB (Class 10) Sandisk Formatted MS-DOS (FAT32) >
Never works, throws error (only works in external card reader)
Old SDHC 32GB (Class 10) Sandisk Formatted MS-DOS (FAT32) >
Doesn’t even throw an error. Eventually becomes visible in Disk Utility with limited info, but not in finder (only works in external card reader)