Linux – Run a Script Before Shutdown with Systemd

Note: Briefly formulated question can be found at the bottom.

I have a script which makes a backup of my files onto an external USB drive. Executing it can take a while, depending on the amount of new data I've created. I want to run it automatically on every shutdown of the system.

I'm using fedora 23 with latest updates (systemd).

I tried to achieve this in several ways, but I couldn't get it to work.

Long running process with StopExec

autobackup.service:

[Unit]
Description=Slow backup script
Requires=local-fs.target

[Service]
ExecStart=/bin/true
ExecStop=/etc/systemd/system/do_backup.sh
Type=oneshot
RemainAfterExit=yes

[Install]
WantedBy=multiuser.target

I activated it with systemctl enable autobackup.service and started it with systemctl start autobackup.service.

Part of my boot log (journalctl -b-1):

Dez 22 17:45:27 localhost systemd[1]: Unmounted /mnt/BACKUP.
Dez 22 17:45:27 localhost do_backup.sh[4296]: At subvol /home/BACKUP.2015_12_22-17_45_25
Dez 22 17:45:27 localhost do_backup.sh[4296]: ERROR: parent subvol is not reachable from inside the root subvol.
Dez 22 17:45:27 localhost do_backup.sh[4296]: At snapshot BACKUP.2015_12_22-17_45_25
Dez 22 17:45:27 localhost do_backup.sh[4296]: ERROR: failed to dump stream. Broken pipe
Dez 22 17:45:27 localhost systemd[1]: autobackup.service: Control process exited, code=exited status=1
Dez 22 17:45:27 localhost systemd[1]: Stopped Slow backup script.
Dez 22 17:45:27 localhost systemd[1]: autobackup.service: Unit entered failed state.

Note that I didn't shorten anything in between, it really unmounted /mnt/BACKUP just before the script started, funny coincidence..

Before shutdown.target

autobackup.service:

[Unit]
Description=Slow backup script
DefaultDependencies=no
Before=shutdown.target

[Service]
ExecStart=/etc/systemd/system/do_backup.sh
Type=oneshot

systemctl edit shutdown.target

[Unit]
Requires=autobackup.service

Output is basically the same.

The Problem

I think the problem is that systemd starts my script in parallel with all the other shutdown scripts in both cases, which unmount BACKUP and deactivate the pipe infrastructure (another error I sometimes got, when the unmount wasn't fast enough).

The Question

How can I teach systemd to start my script first on shutdown, wait until it exits and then start the rest of the shutdown scripts/targets/units/whatever?