Add backup by SERIAL method documentation

docs/deployment/image-backup.rst

@@ -8,6 +8,38 @@ Backing up disk images can still be efficient with Borg because its `deduplicati
 technique makes sure only the modified parts of the file are stored. Borg also has
 optional simple sparse file support for extract.
 
+It is of utmost importancy to pin down the disk you want to backup.
+You need to use the SERIAL for that. 
+Use: 
+
+.. code-block:: bash
+
+    # You can find the short disk serial by:
+    # udevadm info --query=property --name=nvme1n1 | grep ID_SERIAL_SHORT | cut -d '=' -f 2
+
+    DISK_SERIAL="7VS0224F"
+    DISK_ID=$(readlink -f /dev/disk/by-id/*"${DISK_SERIAL}") # Returns /dev/nvme1n1
+
+    mapfile -t PARTITIONS < <(lsblk -o NAME,TYPE -p -n -l "$DISK_ID" | awk '$2 == "part" {print $1}')
+    echo "Partitions of $DISK_ID:"
+    echo "${PARTITIONS[@]}"
+    echo "Disk Identifier: $DISK_ID"
+
+    # Use the following line to perform a borg backup for the full disk:
+    # borg create --read-special /path/to/repo::{now} "$DISK_ID"
+
+    # Use the following to perform a borg backup for all partitions of the disk
+    # borg create --read-special /path/to/repo::{now} "${PARTITIONS[@]}"
+
+    # Example output:
+    # Partitions of /dev/nvme1n1:
+    # /dev/nvme1n1p1
+    # /dev/nvme1n1p2
+    # /dev/nvme1n1p3
+    # Disk Identifier: /dev/nvme1n1
+    # borg create --read-special /path/to/repo::{now} /dev/nvme1n1
+    # borg create --read-special /path/to/repo::{now} /dev/nvme1n1p1 /dev/nvme1n1p2 /dev/nvme1n1p3
+
 Decreasing the size of image backups
 ------------------------------------
 
@@ -116,4 +148,4 @@ way to create application-consistent backups.
 
 Borg doesn't intend to address these issues due to their huge complexity and
 platform/software dependency. Combining Borg with the mechanisms provided by the platform
-(snapshots, hypervisor features) will be the best approach to start tackling them.
+(snapshots, hypervisor features) will be the best approach to start tackling them.