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I created a folder using sudo mkdir /mnt/external/ and then mount an external hdd to it:

$ mount -t ext2 /dev/sdd1 /mnt/external

I have an application writing backups to that mounted drive. Then, for fail test I simply unplug the external drive but the the application still keeps writing backups to that folder.

This is a problem because /mnt/external/ folder now becomes a regular folder on the regular hard drive and the application fills up the actual hard drive which is running the OS.

How can I make sure that the application only writes to the mount point if the drive is currently mounted there?

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With the drive unmounted,

$ sudo chmod a-rwx /mnt/external

A directory doesn't need to be readable or writable to have a filesystem mounted there. When the filesystem is mounted, the permissions of /mnt/external come from the root directory of that filesystem. When no filesystem is mounted there, they come from the filesystem containing /mnt (probably the system's root filesystem).

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  • I have just tested that and confirmed that it works as you suggested. Setting the permissions a-rwx on that folder stopped the backups being written there. Thank you. Jan 16, 2014 at 21:19
  • To clarify, you should remove any existing files from /mnt/external before the chmod. Otherwise, they'll still be there, inaccessible but taking up space.
    – cjm
    Jan 16, 2014 at 21:56
  • yeap i certainly did that. Jan 17, 2014 at 4:57

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