diff --git a/.env.example b/.env.example
index aba8d3e..c1e2cdf 100644
--- a/.env.example
+++ b/.env.example
@@ -32,3 +32,13 @@ PGADMIN_DEFAULT_PASSWORD=replace-with-a-third-independent-random-password
 
 # Optional image override. Keep the same major version when upgrading an existing volume.
 POSTGRES_IMAGE=postgres:17.11-alpine
+
+# Where the two big data sets live. Leave both unset to keep them in Docker volumes, under
+# /var/lib/docker. To use a larger disk, give an absolute path to a directory ON it - a
+# subdirectory, never the mount point itself, which on ext4 holds lost+found and makes PostgreSQL
+# refuse to start. Create the directories and set their owner first; see "Storing the data on
+# another disk" in the README.
+#   POSTGRES_DATA_PATH=/mnt/bigdisk/postgres    owner 70:70, mode 700
+#   MINIO_DATA_PATH=/mnt/bigdisk/minio          owner 1000:1000
+POSTGRES_DATA_PATH=
+MINIO_DATA_PATH=
diff --git a/README.md b/README.md
index babbc6e..c0b4363 100644
--- a/README.md
+++ b/README.md
@@ -93,6 +93,53 @@ intended.
 Upgrade PostgreSQL one major version at a time using the PostgreSQL upgrade procedure. Updating an
 image tag alone does not migrate an existing database volume.
 
+## Storing the data on another disk
+
+By default PostgreSQL and MinIO keep their data in the Docker volumes `postgres-data` and
+`minio-data`, which live under `/var/lib/docker` on the system disk. To use a larger disk, mount it
+on the host and point the two variables at directories on it:
+
+```env
+POSTGRES_DATA_PATH=/mnt/bigdisk/postgres
+MINIO_DATA_PATH=/mnt/bigdisk/minio
+```
+
+The two are independent: set one or both. Unset (or empty) means the named volume, as before.
+
+1. Create the directories **inside** the disk, not at its mount point, and give them to the user the
+   container runs as. On ext4 the mount point holds `lost+found`, and PostgreSQL will not start in
+   a directory that is not empty.
+
+   ```sh
+   sudo mkdir -p /mnt/bigdisk/postgres /mnt/bigdisk/minio
+   sudo chown 70:70 /mnt/bigdisk/postgres && sudo chmod 700 /mnt/bigdisk/postgres   # postgres (alpine)
+   sudo chown 1000:1000 /mnt/bigdisk/minio                                          # MinIO, see minio.Dockerfile
+   ```
+
+2. **A new installation** needs nothing more: set the variables and `docker compose up -d --build`.
+
+3. **An installation that already holds data** must copy it first. Do not use `-v` anywhere:
+
+   ```sh
+   docker compose down
+   docker run --rm -v humainflow-data-stack_postgres-data:/from -v /mnt/bigdisk/postgres:/to alpine cp -a /from/. /to/
+   docker run --rm -v humainflow-data-stack_minio-data:/from    -v /mnt/bigdisk/minio:/to    alpine cp -a /from/. /to/
+   # set the two variables in .env, then
+   docker compose up -d --build
+   ```
+
+   The volume names carry the project name from `name:` in the compose file. Keep the old volumes
+   until everything works, and remove them only then.
+
+Cautions:
+
+- Use a local or block disk. PostgreSQL on a network filesystem such as NFS risks corruption.
+- Mount the disk at boot (`/etc/fstab`, with `nofail`) and start Docker after it. If the disk is
+  missing when the stack starts, Docker creates the directory on the system disk and PostgreSQL
+  starts **empty** - it looks like the data is gone, and it is only somewhere else.
+- A backup of `POSTGRES_DATA_PATH` taken while PostgreSQL runs is not consistent; use `pg_dump` or a
+  proper base backup.
+
 ## Upgrading from the four-name layout
 
 An earlier version used `POSTGRES_DOMAIN`, `MINIO_API_DOMAIN`, `MINIO_CONSOLE_DOMAIN` and
diff --git a/docker-compose.yml b/docker-compose.yml
index c48a0cf..e7b339a 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -20,7 +20,8 @@ services:
       - -c
       - password_encryption=scram-sha-256
     volumes:
-      - postgres-data:/var/lib/postgresql/data
+      # A host directory when POSTGRES_DATA_PATH is set (see the README), the named volume when not.
+      - ${POSTGRES_DATA_PATH:-postgres-data}:/var/lib/postgresql/data
     expose:
       - "5432"
     networks:
@@ -49,7 +50,8 @@ services:
       MINIO_BROWSER_REDIRECT_URL: https://${DATA_DOMAIN}:${CONSOLE_PUBLIC_PORT:-9443}
     command: ["server", "/data", "--console-address", ":9001"]
     volumes:
-      - minio-data:/data
+      # A host directory when MINIO_DATA_PATH is set (see the README), the named volume when not.
+      - ${MINIO_DATA_PATH:-minio-data}:/data
     expose:
       - "9000"
       - "9001"