I'm setting up an embedded device running Debian Jessie to shut down when a UPS experiences a power failure. Unfortunately, the device's bootloader restarts the OS whenever it quits, so shutdown/halt commands are equivalent to restart. 

My main concern is protecting the data on the attached hard disks. I think my best option is to kill as many tasks as possible and then wait for the power to cut. `systemctl isolate emergency.target` seems close to what I want, except it leaves the disks mounted. Is there a way to basically get Systemd to `final.target` without an actual shutdown/halt command?