root@raspberrypi:/dev# mkfs.ext2 /media/F839-11B6/
mke2fs 1.42.5 (29-Jul-2012)
/media/F839-11B6/ is not a block special device.
Proceed anyway? (y,n) y
mkfs.ext2: Device size reported to be zero.  Invalid partition specified, or
        partition table wasn't reread after running fdisk, due to
        a modified partition being busy and in use.  You may need to reboot
        to re-read your partition table.