Is this a camera card?
Camera makers never agreed on SD card formats like computer makers did for their storage devices. Some camera makers "did their own thing."
Some camera cards show up in the FInder just like any external drive; other never show but can be accessed. Cards from the Nikon cameras I use never show in the Finder but the contents are fully accessible in two ways, both free:
1) Image Capture is included with Mac OS in your Applications folder. It reads camera cards quite nicer and allows importing selected images or everything on the card. Apple Support article here:
Transfer images in Image Capture on Mac - Apple Support
2) Use the image management software offered by you camera maker.
Both methods act as translator for the Finder to read those goofy card formats