I was making a piratebox and the USB needed to be formatted in FAT32, MBR format. Apple’s disk utility wasn’t co-operating, so I moved over to Terminal.

Terminal

In Terminal, search for your USB using the command:

Find your USB name and disk number, then enter the following command (be careful you don’t select the wrong drive! It will erase it for good.):

diskutil eraseDisk FAT32 DISKNAME MBR /dev/disk#

*changing out the bolded text.

Example:
diskutil eraseDisk FAT32 IsaacsUSB MBR /dev/disk2

This will make your USB a FAT32 formatted drive.

“FAT” stands for File Allocation Table, which is a computer file system architecture. It offers good performance even in lightweight implementations. It is supported for compatibility reasons by nearly all currently developed operating systems for personal computers and many mobile devices, thus is a well-suited format for data exchange between computers and devices of almost any type and age from 1981 up to the present.