$ sudo umount /dev/md0
umount: /media/vault: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
$ sudo fuser /dev/md0
$ sudo lsof | grep md0
md0_raid6   325       root  cwd       DIR              8,130      4096          2 /
md0_raid6   325       root  rtd       DIR              8,130      4096          2 /
md0_raid6   325       root  txt   unknown                                         /proc/325/exe
jbd2/md0-   578       root  cwd       DIR              8,130      4096          2 /
jbd2/md0-   578       root  rtd       DIR              8,130      4096          2 /
jbd2/md0-   578       root  txt   unknown                                         /proc/578/exe
bash       1714    barrett  cwd       DIR               0,14         0      20526 /sys/devices/virtual/block/md0/md
screen    13472    barrett  cwd       DIR               0,14         0      20526 /sys/devices/virtual/block/md0/md
screen    13473    barrett  cwd       DIR               0,14         0      20526 /sys/devices/virtual/block/md0/md