He intentado matar el proceso:
sam@sam-desktop:~$ ps -aux|grep sda
Warning: bad ps syntax, perhaps a bogus '-'? See http://procps.sf.net/faq.html
root 2898 0.0 0.0 0 0 ? S 11:39 0:00 [jbd2/sda6-8]
root 2899 0.0 0.0 2300 716 ? D 11:39 0:00 mount -t ext4 -o uhelper=udisks,nodev,nosuid /dev/sda6 /media/634bad56-5543-40fe-843b-cd31f4a95dba_
sam 2973 0.0 0.0 3328 876 pts/0 S+ 14:13 0:00 grep --color=auto sda
sam@sam-desktop:~$ sudo kill -9 2898
sam@sam-desktop:~$ sudo kill -9 2899
sam@sam-desktop:~$ sudo killall -9 2898
2898: no process found
sam@sam-desktop:~$ sudo killall -9 2899
2899: no process found
sam@sam-desktop:~$ ps -aux|grep sda
Warning: bad ps syntax, perhaps a bogus '-'? See http://procps.sf.net/faq.html
root 2898 0.0 0.0 0 0 ? S 11:39 0:00 [jbd2/sda6-8]
root 2899 0.0 0.0 2300 716 ? D 11:39 0:00 mount -t ext4 -o uhelper=udisks,nodev,nosuid /dev/sda6 /media/634bad56-5543-40fe-843b-cd31f4a95dba_
sam 2987 0.0 0.0 3328 872 pts/0 S+ 14:22 0:00 grep --color=auto sda
sam@sam-desktop:~$
Después de las sugerencias que probé:
sam@sam-desktop:~$ sudo umount -f /media/634bad56-5543-40fe-843b-cd31f4a95dba_
umount2: Invalid argument
umount: /media/634bad56-5543-40fe-843b-cd31f4a95dba_: not mounted
sam@sam-desktop:~$ sudo umount -l /media/634bad56-5543-40fe-843b-cd31f4a95dba_
umount: /media/634bad56-5543-40fe-843b-cd31f4a95dba_: not mounted
sam@sam-desktop:~$