systemd keeps unmounting a removable drive

mnt-zeno.mount was created by systemd-fstab-generator. According to Jonathan de Boyne Pollard's explanation on debian-user mailing list:

[systemd-fstab-generator is] a program that reads /etc/fstab at boot time and generates units that translate fstab records to the systemd way of doing things [.....]

The systemd way of doing things is mount and device units, per the systemd.mount(5) and systemd.device(5) manual pages. In the raw systemd way of doing things, there's a device unit named "dev-sde1.device" which is a base requirement for a mount unit named "media-lumix\x2dphotos.mount".

After altering fstab one should either run systemctl daemon-reload (this makes systemd to reparse /etc/fstab and pick up the changes) or reboot.

For people who come here via a google search (like me), the developers have known about this problem for almost 5(!) years as of writing this and still nobody tries to fix it. https://github.com/systemd/systemd/issues/1741

Workaround: Often, restarting the systemd daemon via systemctl daemon-reload solves the problem.