Você deve desmontar pelo nome do diretório, não pelo nome do dispositivo.
man umount:
The umount command detaches the mentioned file system(s) from the file
hierarchy. A file system is specified by giving the directory where it
has been mounted. Giving the special device on which the file system
lives may also work, but is obsolete, mainly because it will fail in
case this device was mounted on more than one directory.