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Back up any important data from the drive. Open Disk Utility from the Utilities folder. Select your USB drive from the list on the left. Click the "Erase" tab. Select "MS-DOS (FAT)" from the "Format" or "Volume Format" menu. Give the USB drive a name. Click "Erase" to begin the format. Test your USB drive.
Formatting the drive will erase everything on it, so make sure any important files are safely copied to another location before proceeding. You can find this folder in your Applications folder. If your USB drive is not appearing, try a different USB port. If you can't get it to appear in any port or on another computer, it may be broken. This will display the formatting options for the USB drive. Even though it is listed as "FAT," this is actually the FAT32 file system. Note that FAT32 only supports drives up to 32 GB in size, as well as files up to 4 GB. If you have a larger drive or need to transfer larger files, consider select the more modern "exFAT" format instead. If you're only going to be using the drive with Mac computers, select "Mac OS Extended (Journaled)." If the drive is larger than 32 GB but you absolutely need FAT32, you can create multiple partitions on the USB drive and format each as a separate FAT32 partition. Click the "Partition" tab and then click the "+" button to create new partitions. Set the size of each to 32 GB or less, and select "MS-DOS (FAT)" from the Format menu for each. Enter a label for the drive into the "Name" field. This name will appear whenever the drive is connected to a computer or device. All of the data on the drive will be deleted and it will be formatted with the FAT32 file system. After the format, you should be able to add and remove files to the USB drive without issue. You'll find the USB drive on your desktop.