Problems formatting & reading/writing Micro SD's on iMac.

A few things to note if you are planning to use those microSD cards in cameras.

• The standard filesystem for SDHC / microSDHC cards is FAT32. (Not HFS+, APFS, NTFS, or exFAT.)
• The standard filesystem for SDXC / microSDXC cards is exFAT. (Not HFS+, APFS, NTFS, or FAT32.)
• Many photographers format cards only in the cameras that are going to use them.
• If you must format a card in a computer, the SD Association recommends using their special formatting utility rather than the standard formatting tools in macOS and Windows.

https://www.sdcard.org/downloads/formatter/

By RPi, do you mean a Raspberry Pi single-board computer? I found an article claiming that the only formats that the Raspberry Pi boot loader does not support exFAT, so if you were using a microSDXC card (which you're not), you would need to reformat it using FAT32. (Some FAT32 formatters do not allow creating volumes larger than 32 GB.)

Back to the "16 GB SanDisk" cards that showed 16 GB when plugged in, but only 268 MB after a Disk Utility erase. I've heard stories of counterfeit cards that have storage than they are advertised to have, and that are even rigged to make a computer think they have the larger amount. You write your 16 GB of photos and videos, and everything seems fine, but when you go to read things back, you find that the data you wrote first has been overwritten by the data you wrote later …

This is often cited as a cautionary tale for never buying cards from an unknown vendor. Are you sure these "16 GB SanDisk cards" you have are the genuine article?

jackmorton wrote:

I've got an off-brand adapter with USB & SD/MicroSD slots plugged into one of the USB-C's.

By "adapter", do you mean a USB-C dock or hub?

There are microSD-to-SD adapters (often bundled with microSD cards) where you slot the microSD card into the adapter, and then you put the microSD + adapter assembly into a SD card reader …

When I say they are evil, I mean that they will stop working or corrupt images, it has nothing to do with the format. Get a full size SD card for the camera and the problem goes away.

Maybe Canon's SD slots don't put enough pressure on the card to maintain good contact.

Around the Canon Community forums, MicroSD's are evil.

I suggest you try to find a reader that reads them without an adapter.