Prevent Lubuntu 22.04 (SDDM and LXQt) from leaving an external drive mounted incorrectly for other users

An external USB disk drive is connected permanently to my family’s PC that currently has Lubuntu 22.04 installed. There are several user accounts on this multi-user single-seat machine. If a user does not unmount the external drive before logging out, when another user logs in, the external drive is still mounted with the priviledges of the previous user. In the days when Lubuntu used LightDM and LXDE, I devised a scheme to unmount automatically the external drive when each user logged out (see an earlier post). However, that method is no longer possible now Lubuntu no longer uses LightDM. Therefore I devised a different scheme. This time, the external drive is unmounted automatically every time a user logs in, and udisks2 automatically mounts it for the current user. Below I explain the new scheme. In the example below the external USB drive happens to be an NTFS drive and have the label ‘FREECOM HDD’. The machine actually has several user accounts but, for the sake of brevity, the commands for only two users are shown. All the steps shown below are performed by the account holder with the ability to use the sudo command to get superuser priviledges.

1. Create a root script to unmount the external USB drive

$ sudo nano /usr/local/sbin/unmount_FREECOM_HDD.sh

The file contains the following lines:

#!/bin/bash
# Unmount the external USB HDD if mounted with another username in the path:
umount /media/*/FREECOM\ HDD 2>/dev/null

2. Make sure the script has the correct priviledges

$ sudo chmod 755 /usr/local/sbin/unmount_FREECOM_HDD.sh

3. Give each user permission to use the sudo command to run the abovementioned script

$ sudo visudo -f /etc/sudoers.d/unmount_FREECOM_HDD

Add a line for each user:

fitzcarraldo ALL=NOPASSWD: /usr/local/sbin/unmount_FREECOM_HDD.sh
molly ALL=NOPASSWD: /usr/local/sbin/unmount_FREECOM_HDD.sh

4. Create an autostart Desktop Configuration File for each user

$ sudo nano "/home/fitzcarraldo/.config/autostart/Unmount FREECOM HDD.desktop"
$ sudo nano "/home/molly/.config/autostart/Unmount FREECOM HDD.desktop"

Each user’s autostart file should contain the following:

[Desktop Entry]
Exec=sudo /usr/local/sbin/unmount_FREECOM_HDD.sh
Name=Unmount FREECOM HDD
OnlyShowIn=LXQt;
Type=Application
Version=1.0

Make sure the autostart files have the correct ownership and priviledges

$ sudo chown fitzcarraldo:fitzcarraldo "/home/fitzcarraldo/.config/autostart/Unmount FREECOM HDD.desktop"
$ sudo chmod 664 "/home/fitzcarraldo/.config/autostart/Unmount FREECOM HDD.desktop"
$ sudo chown molly:molly "/home/molly/.config/autostart/Unmount FREECOM HDD.desktop"
$ sudo chmod 664 "/home/molly/.config/autostart/Unmount FREECOM HDD.desktop"

(If each user logs in to their LXQt Desktop, clicks on the Application Menu icon and selects ‘Preferences’ > ‘LXQt Settings’ > ‘Session Settings’ and clicks on Autostart’, there should be the ticked entry ‘Unmount FREECOM HDD’ in the ‘LXQt Autostart’ section.)

5. Reboot and the scheme should take effect.