USBMount not mounting problem

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copied from: https://codeyarns.com/tech/2019-05-18-usbmount-not-mounting-problem.html

I had a portable harddisk connected using USB to my Raspberry Pi B+ Rev3 running Raspbian 9. The harddisk had two partitions: NTFS and ext4. I had usbmount installed and configured as described here. When Raspbian is booted up with the harddisk already plugged in, I could see using sudo fdisk -l that the ext4 partition was visible. So, usbmount should have automatically mounted this ext4 partition to /media/usb0, but it was not doing that.

I had verbose logging enabled for usbmount as described here. When I searched the system log I could see that usbmount was actually mounting the ext4 partition, but surprisingly it was not visible at /media/usb0:

$ cat /var/log/syslog | grep usbmount
980:May 18 11:42:28 my-pi usbmount[373]: /dev/sda2 contains filesystem type ext4
989:May 18 11:42:28 my-pi usbmount[373]: mountpoint /media/usb0 is available for /dev/sda2
991:May 18 11:42:28 my-pi usbmount[373]: executing command: mount -text4 -osync,noexec,nodev,noatime,nodiratime /dev/sda2 /media/usb0
1010:May 18 11:42:28 my-pi usbmount[373]: executing command: run-parts /etc/usbmount/mount.d
1013:May 18 11:42:28 my-pi usbmount[373]: usbmount execution finished
Solution
Turned out that systemd was the culprit. Open your /usr/lib/systemd/system/systemd-udevd.service and see if you have either a PrivateMounts=yes line (in Ubuntu 20.04 or later) or a MountFlags=slave line (in Ubuntu 18.04 or older). So, the mount operation of usbmount was mounting into the namespace of systemd and not to the user.

To change this mount option, run this command:

$ sudo systemctl edit systemd-udevd
Add these lines to the empty file that is opened and save that file:

(for Ubuntu 20.04 or later)

[Service]
PrivateMounts=no
(for Ubuntu 18.04 or older)

[Service]
MountFlags=shared
I found that this created the file /etc/systemd/system/systemd-udevd.service.d/override.conf with the above lines.

Restart systemd:

$ sudo systemctl daemon-reload
$ sudo service systemd-udevd --full-restart
Now, reinsert the storage device or restart the Pi.

After the restart, I found that usbmount had automatically mounted my ext4 partition.

[undefined (1j9rwias)]

bug still exists in 2022-04-04-raspios-bullseye-armhf-lite.img