If you need to Reformat a USB drive or external hard drive, you can do this simply using Disk Utility .

Warning: This Will Delete Everything On Your USB/External Hard Drive.

1. Open Disk Utility .
    
2. Select The Drive that you wish to reformat and click Erase.
   
   
    
3. Type in a Name for the newly formatted drive, and select the format MS-DOS (FAT). Click Erase.
   
   
    

When this process is successfully completed the disk will be reformatted.