Most external hard drives and USB drives can be used with Mac computers if you format them appropriately with Disk Utility.
Step 1. Connect your USB drive to your Mac
Step 2. Open the Applications folder and click on Utilities
Step 3. Select "Disk Utility"
The Disk Utility window will open.
Step 4. Click on the name of the USB drive
It will appear in the left pane of the Disk Utility window.
Step 5. Click "Erase" at the top of the window
Step 6. Open the "Format" menu
Step 7. Select "Mac OS Extended (Journaled)" or another file system format
This option will ensure that your USB drive is fully compatible with your Mac, because by default most USB drives are pre-formatted to run on Windows.
Step 8. Enter a name for the USB drive in the Name line
Step 9. Click "Erase" in the lower right corner
Step 10. Click Erase again when prompted
The USB drive is formatted and ready to use on your Mac.