SD Card on Mac Showing Empty, but Still Holding Data

Maybe you can log out and log in Apple account, or shut down your Mac completely.
Then open Terminal to unhide files, type defaults write com.apple.finder AppleShowAllFiles true; killall Finder.

If all of these fail, you’d better consider using some data recovery tools like iBoysoft Data Recovery for Mac. It will scan the SD card and recover all gone files.

It’s also possible that the card itself is damaged:

(1) Try a different device:

Insert the SD card into another computer or a camera to see if the files are accessible.

(2) Backup first:

If you manage to access the files, copy them to another location immediately.

(3)Reformat as a last resort:

If the card is still unresponsive and you’ve recovered your data, you might need to reformat it to use it again. Just make sure you select the appropriate file system (e.g., exFAT or FAT32) in Disk Utility.

https://iboysoft.com/howto/cant-eject-external-hard-drive-mac.html