root@t510 0 /usr/lib/systemd/system # cat external.service
[Unit]
Description=/external
DefaultDependencies=false
Wants=-.mount
[Service]
Type=simple
ConditionFileIsExecutable=/usr/lib/systemd/scripts/external-start
ConditionFileIsExecutable=/usr/lib/systemd/scripts/external-stop
ExecStart=/usr/lib/systemd/scripts/external-start
ExecStop=/usr/lib/systemd/scripts/external-stop
TimeoutSec=10
RemainAfterExit=yes

[Install]
WantedBy=local-fs.target
Also=systemd-udev-settle.service