Keep the following in mind when formatting removable media:
Close and quit the file manager window.
File Manager automatically displays a formatting window when you insert an unformatted media. To avoid the window, quit from File Manager. If you prefer to keep File Manager open, quit the formatting window when it appears.
Volume manager (vold) mounts file systems automatically so you might have to unmount media before you can format it, if it contains an existing file system.