USBMount not mounting problem

Problem

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. It was having a MountFlags=slave in its systemd-udevd.service file. 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 file that is opened:

[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 or your Pi:

$ sudo systemctl daemon-reload
$ sudo service systemd-udevd --full-restart

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

References:

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