See this blog. Might be a few reasons:
- Device is busy with open files. Use “lsof /mnt/test” to show which file is opened by which process (assume /mnt/test is the mounting point). Then use “kill -9 pids” to stop the processes.
- NFS kernel server is exporting directories on the device. Use “/etc/init.d/nfs-kernel-server stop” to stop the server.
- Loopback is set backed by a file. Use “losetup -a” to see which loopback device and the file is open then use “umount file” to umount the file and “losetup -d <loopback>” to stop the loopback.
- A swap file exists on the path. See “/proc/swap”. Then stop the swap.
- Disk failure.