diff --git a/README.md b/README.md
index f82360b..05a1efc 100644
--- a/README.md
+++ b/README.md
@@ -112,15 +112,20 @@ docker run --rm -v /var/lib/sfdupes:/var/lib/sfdupes sfdupes report > dupes.tsv
 Run `scan` as root, so that it can read every file. `scan` warns about a
 directory it cannot read, and about a new or changed file it cannot read. It
 skips the path and deletes the records at or below it (see "Rules for the
-walk"). As the file `/etc/cron.d/sfdupes`:
+walk").
+
+An unmounted disk leaves its mount point empty, so `scan` deletes every record
+under it, and the next scan reads every file on the disk again. For each other
+disk mounted below `/srv`, add `&& mountpoint -q` and its mount point after the
+first test. As the file `/etc/cron.d/sfdupes`:
 
 ```
-30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
+30 3 * * * root if mountpoint -q /srv; then /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log; else echo "a disk is not mounted; scan of /srv skipped"; fi
 ```
 
 - The first scan creates `/var/lib/sfdupes/db.sqlite` and its directory. To keep
-  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` before the
-  command on the same line.
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
+  before `/usr/local/bin/sfdupes`.
 - `scan` writes nothing to stdout. Its stderr, appended here to
   `/var/log/sfdupes.log`, holds:
     - one line naming the database;
diff --git a/TODO.md b/TODO.md
index 94e5c67..948d427 100644
--- a/TODO.md
+++ b/TODO.md
@@ -29,6 +29,9 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- `README.md` says what a disk that is not mounted does to the database. The
+  daily `cron` line runs `scan` only when `mountpoint` reports `/srv` as mounted
+  (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/175)
 - The usage line of `scan` no longer ends in `[flags]`. It is the line
   `README.md` shows under "Subcommands" (2026-11-11,
   https://git.eeqj.de/sneak/sfdupes/issues/169)