docs: automated-local: support multiple backup drives, fixes borgbackup#9834

The udev rule matched a single partition table UUID, so only the first
backup drive started the backup service, although run.sh accepts every
filesystem UUID listed in backup.disks.

Match on the filesystem UUID (the same UUID as in backup.disks) instead,
document one rule line per backup drive (or "|" alternatives) and add
the udev rule step to "Adding backup hard drives".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: ThomasWaldmann

docs/deployment/automated-local.rst

@@ -14,8 +14,8 @@ systemd and udev.
 Overview
 --------
 
-A udev rule is created to trigger on the addition of block devices. The rule contains a tag
-that triggers systemd to start a one-shot service. The one-shot service executes a script in
+A udev rule is created to trigger on the addition of a filesystem on one of your registered
+backup drives. The rule contains a tag that triggers systemd to start a one-shot service. The one-shot service executes a script in
 the standard systemd service environment, which automatically captures stdout/stderr and
 logs it to the journal.
 
@@ -29,13 +29,16 @@ Configuring the system
 First, create the ``/etc/backups`` directory (as root).
 All configuration goes into this directory.
 
-Find out the ID of the partition table of your backup disk (here assumed to be /dev/sdz)::
+Then, create an empty ``/etc/backups/80-backup.rules`` file. It will contain one line per
+backup drive, which you will add when adding backup drives (see below). Each line looks like
+this (all on one line)::
 
-    lsblk --fs -o +PTUUID /dev/sdz
+    ACTION=="add", SUBSYSTEM=="block", ENV{ID_FS_UUID}=="<filesystem UUID>", TAG+="systemd", ENV{SYSTEMD_WANTS}+="automatic-backup.service"
 
-Then, create ``/etc/backups/80-backup.rules`` with the following content (all on one line)::
-
-    ACTION=="add", SUBSYSTEM=="block", ENV{ID_PART_TABLE_UUID}=="<the PTUUID you just noted>", TAG+="systemd", ENV{SYSTEMD_WANTS}+="automatic-backup.service"
+The rule matches the filesystem on the backup drive by its UUID, which is the same UUID
+that you will also record in ``/etc/backups/backup.disks``. A single line can also match
+several backup drives, if you separate their UUIDs with ``|``, e.g.
+``ENV{ID_FS_UUID}=="<UUID 1>|<UUID 2>"``.
 
 The "systemd" tag in conjunction with the SYSTEMD_WANTS environment variable has systemd
 launch the "automatic-backup" service, which we will create next, as the
@@ -54,12 +57,7 @@ modify it to suit your needs (e.g., more backup sets, dumping databases, etc.).
 
     #!/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.
+    # Give the system a moment to settle after the backup drive was connected.
     sleep 5
 
     #
@@ -186,6 +184,13 @@ Find the UUID of the filesystem on which backups should be stored::
 
 Record the UUID in the ``/etc/backups/backup.disks`` file.
 
+Add a line for this UUID to ``/etc/backups/80-backup.rules`` (see above), so that connecting
+the drive starts the ``automatic-backup`` service, and reload the udev rules::
+
+    udevadm control --reload
+
+Every backup drive needs both: its UUID in ``backup.disks`` and a matching udev rule.
+
 Mount the drive at /mnt/backup.
 
 Create a Borg repository at the location indicated by ``TARGET``::