I followed the guide found here, however with a few modifications.

Notably, I did not encrypt the borg repository, and heavily modified the backup script.

#!/bin/bash -ue

# The udev rule is not terribly accurate and may trigger our service before
# the kernel has finished probing partitions. Sleep for a bit to ensure
# the kernel is done.
#
# This can be avoided by using a more precise udev rule, e.g. matching
# a specific hardware path and partition.
sleep 5

#
# Script configuration
#

# The backup partition is mounted there
MOUNTPOINT=/mnt/external

# This is the location of the Borg repository
TARGET=$MOUNTPOINT/backups/backups.borg

# Archive name schema
DATE=$(date '+%Y-%m-%d-%H-%M-%S')-$(hostname)

# This is the file that will later contain UUIDs of registered backup drives
DISKS=/etc/backups/backup.disk

# Find whether the connected block device is a backup drive
for uuid in $(lsblk --noheadings --list --output uuid)
do
        if grep --quiet --fixed-strings $uuid $DISKS; then
                break
        fi
        uuid=
done

if [ ! $uuid ]; then
        echo "No backup disk found, exiting"
        exit 0
fi

echo "Disk $uuid is a backup disk"
partition_path=/dev/disk/by-uuid/$uuid
# Mount file system if not already done. This assumes that if something is already

See my other reply here.

See my other reply here.

See my other reply here.