Improper ejection may damage the SD card to make it unreadable on your Mac. You should make sure the SD card reader, the USB port, the USB cable, and the hub are in good condition. Try methods such as manually mounting the SD card, repairing it with Disk Utility, reformating the card, etc., to make the SD card readable again.
The SD card shouldn't eject on its own, the unexpected ejection could be caused by sudden power failure, unstable USB connection, software bugs on your Mac, and others. Then, the Disk Not Ejected Properly error pops up the next time you insert the SD card, and you find the SD card is unreadable.
When the SD card is not readable on Mac, you should first check the connection between the SD card and your computer, the elements including the USB port, SD card reader, USB cable, and the hub if used. Secondly, try to mount the SD card manually in Disk Utility and repair it with First Aid.
The last resort you can try by yourself is to reformat the SD card to solve logical errors on it, this solution may work to make the SD card readable again on your Mac. However, reformatting erases everything on the SD card, thus, you are recommended to recover lost files on the SD card before reformatting, and iBoysoft Data Recovery for Mac is a good consideration to help recover data from unreadable SD cards. Download it for free and give it a shot!
