Wipe everything off a removable disk
If you have a removable disk, like a USB memory stick or an external hard disk, you may sometimes wish to completely remove all of its files and folders. You can do this by formatting the disk - this deletes all of the files on the disk and leaves it empty.
Format a removable disk
Open the Disk Utility application from the dash.
Select the disk you want to wipe from the Storage Devices list.
Make sure that you have selected the correct disk! If you choose the wrong disk, all of the files on the other disk will be deleted!
In the Volumes section, click Unmount Volume. Then click Format Volume.
In the window that pops up, choose a filesystem Type for the disk.
If you use the disk on Windows and Mac OS computers in addition to Linux computers, choose FAT. If you only use it on Windows, NTFS may be a better option. A brief description of the file system type will be presented as a label.
Give the disk a name and click Format to begin wiping the disk.
Once the formatting has finished, safely remove the disk. It should now be blank and ready to use again.
Formatting a disk does not securely delete your files
Formatting a disk is not a completely secure way of wiping all of its data. A formatted disk will not appear to have files on it, but it is possible that special recovery software could retrieve the files. If you need to securely delete the files, you will need to use a command-line utility, such as shred.