Oddly I have an "half sd card" that I can use for extra storage in my old 15 inch work MacBook. Its like an SD card with a food that fits perfectly (almost flush) when iserted into the sd slot. It has a lip so you can remove it easily.

I though I'd have a mountable extra storage.. But ultimately it would sometimes unmount on sleep (you could unplug and replug..) and it was a little slow. If I wanted to use the SD card from my camera, I had to remove it. I replaced it with an external ssd. which is bulkier and corded...

this wasn't the exact thing, but it gives an idea what its like: https://www.amazon.com/Transcend-JetDrive-Storage-Expansion-...