[Unit]
Description=backing up data
OnFailure=systemd-email@%n.service
ConditionPathExists={{backup_to_swappable_destination_path}}

[Service]
Type=oneshot
ExecStart=/usr/bin/bash {{backup_to_swappable_script_path}} {{backup_to_swappable_destination_path}} {{backup_to_swappable_source_path}}