Linuxexit code > 0

umount: target is busy

$umount: /mnt/data: target is busy.

Analysis

You cannot unmount the filesystem because a process is currently using it (has an open file or current working directory there).

Common Triggers

  • Shell open in that directory.
  • Background service writing to a log file there.

Debug Checks

  • $Find using processes: lsof +f -- /mnt/data or fuser -m /mnt/data.

Resolution

1
Kill the processes.
2
Lazy unmount (detach now, clean up later): umount -l /mnt/data.