Connect the SD card to your computer
Use whatever SD card adapter you have to connect your SD card to your computer.
Open Disk Utility
Navigate to Applications > Utilities > Disk Utility
Create a new image from your SD card
Select your SD card in the sidebar and then click New Image.
Name your backup and select a backup location
You can keep the default settings in the Save window. Select a backup location and give your image backup a name.
Safely eject your SD card
Don't forget to safely eject your card before removing it from your computer. You're all done!