#mount
/dev/disk2 on / (hfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
/dev/disk4s1 on /Volumes/USB_Disk (msdos, local, nodev, nosuid, noowners)

#diskutil unmountDisk disk4
Unmount of all volumes on disk4 was successful

#sudo newfs_msdos -F 16 /dev/disk4

#diskutil mount /dev/disk4