de man 8 umount
:
-f Force unmount (in case of an unreachable NFS system).
(Requires kernel 2.1.116 or later.)
-l Lazy unmount. Detach the filesystem from the filesystem hierar-
chy now, and cleanup all references to the filesystem as soon
as it is not busy anymore. (Requires kernel 2.4.11 or later.)
Se sudo umount -f /mnt/1
não funcionar, você pode tentar sudo umount -l /mnt/1
.