Formatting a USB flash drive on your Mac will remove all data on the drive and offer the opportunity to change the file system used, which is how data is organized on the drive. The biggest concern when formatting the flash drive is which file system to use and the answer depends on your expected use of the drive.
The Mac OS Extended, or HFS+, format is the obvious choice if you only plan to use the drive on Mac computers. For the ultimate, cross-system compatibility, the MS-DOS (FAT) system, which is the Mac’s name for FAT32, should be used, but unfortunately, this limits individual file sizes to 4GB. For compatible use among the latest versions of Mac and Windows without the FAT32 file size limit, exFAT is the way to go.
Insert the USB flash drive into your Mac’s USB port.
Click “Finder | Applications | Utilities | Disk Utility.” Alternatively, enter “disk utility” in the search bar and click “Disk Utility” to launch the application.
Click the USB flash drive from the left pane of Disk Utility. Make sure to select the root folder and not an indented entry under it.
Click the “Erase” tab on the right pane.
Click the “Format” drop-down menu and select your preferred file system. Optionally, enter a name for the drive in the “Name” field.
Click “Security Options” if you want to completely erase existing data by overwriting it once, seven times or 35 times. The more times the data is overwritten, the more secure the format, but the longer it takes. Make your selection and click “OK.”
Click “Erase” and then “Erase” again in the confirmation window. Depending on the size of the drive and the security selection, formatting might take a few seconds or several minutes.
After formatting, the existing data will be gone. Make sure to back up any important data before formatting the drive.