diff --git a/Payload_img_design.md b/Payload_img_design.md
new file mode 100644
index 00000000..d37fae5f
--- /dev/null
+++ b/Payload_img_design.md
@@ -0,0 +1,102 @@
+# live-bootstrap
+
+This repository uses [`README.rst`](./README.rst) as the canonical main documentation.
+
+## Kernel-bootstrap `payload.img`
+
+`payload.img` is a raw container disk used in kernel-bootstrap offline mode
+(`--repo` and `--external-sources` are both unset).
+
+### Why not put everything in the initial image?
+
+In kernel-bootstrap mode, the first boot image is consumed by very early
+runtime code before the system reaches the normal bash-based build stage.
+That early stage has tight assumptions about memory layout and file table usage.
+
+When too many distfiles are packed into the initial image, those assumptions can
+be exceeded, which leads to unstable handoff behavior (for example, failures
+around the Fiwix transition in QEMU or on bare metal).
+
+So the design is intentionally split:
+
+- Initial image: only what is required to reach `improve: import_payload`
+- `payload.img`: the rest of offline distfiles
+
+This is not a patch-style workaround. It is a two-phase transport design that
+keeps early boot deterministic and moves bulk data import to a stage where the
+runtime is robust enough to process it safely.
+
+### Why import from an external image and copy into main filesystem?
+
+Because the bootstrap still expects distfiles to end up under the normal local
+path (`/external/distfiles`) for later steps. `payload.img` is used as a
+transport medium only.
+
+The flow is:
+
+1. Boot minimal initial image.
+2. Reach `improve: import_payload`.
+3. Detect the payload disk by magic (`LBPAYLD1`) across detected block devices.
+4. Copy payload files into `/external/distfiles`.
+5. Continue the build exactly as if files had been present locally all along.
+
+### Format
+
+- Magic: `LBPAYLD1` (8 bytes)
+- Then: little-endian `u32` file count
+- Repeated entries:
+  - little-endian `u32` name length
+  - little-endian `u32` file size
+  - file name bytes (no terminator)
+  - file bytes
+
+The importer probes detected block devices and selects the one with magic `LBPAYLD1`.
+
+### Manual creation without Python
+
+Prepare `payload.list` as:
+
+```text
+<archive-name> <absolute-path-to-archive>
+```
+
+Then:
+
+```sh
+cat > make-payload.sh <<'SH'
+#!/bin/sh
+set -e
+out="${1:-payload.img}"
+list="${2:-payload.list}"
+
+write_u32le() {
+  v="$1"
+  printf '%08x' "$v" | sed -E 's/(..)(..)(..)(..)/\4\3\2\1/' | xxd -r -p
+}
+
+count="$(wc -l < "${list}" | tr -d ' ')"
+: > "${out}"
+printf 'LBPAYLD1' >> "${out}"
+write_u32le "${count}" >> "${out}"
+
+while read -r name path; do
+  [ -n "${name}" ] || continue
+  size="$(wc -c < "${path}" | tr -d ' ')"
+  write_u32le "${#name}" >> "${out}"
+  write_u32le "${size}" >> "${out}"
+  printf '%s' "${name}" >> "${out}"
+  cat "${path}" >> "${out}"
+done < "${list}"
+SH
+chmod +x make-payload.sh
+./make-payload.sh payload.img payload.list
+```
+
+Attach `payload.img` as an extra raw disk in QEMU, or as the second disk on bare metal.
+
+### When it is used
+
+- Used in kernel-bootstrap offline mode.
+- Not used when `--repo` or `--external-sources` is provided.
+- `--build-guix-also` increases payload contents (includes post-early `steps-guix`
+  sources), but does not change the mechanism.
diff --git a/README.rst b/README.rst
index 154c860c..decb33c7 100644
--- a/README.rst
+++ b/README.rst
@@ -63,17 +63,78 @@ Without using Python:
 
         * *Only* copy distfiles listed in ``sources`` files for ``build:`` steps
           manifested before ``improve: get_network`` into this disk.
-      * Optionally (if you don't do this, distfiles will be network downloaded):
+      * In kernel-bootstrap offline mode (no ``--repo`` and no
+        ``--external-sources``), use the second image as ``payload.img``.
+        ``payload.img`` is a raw container (not a filesystem) used to carry the
+        distfiles that are not needed before ``improve: import_payload``.
+        In other words, the first image only carries the minimal set needed to
+        reach the importer; the rest of the offline distfiles live in payload.
 
-        * On the second image, create an MSDOS partition table and one ext3
-          partition.
-        * Copy ``distfiles/`` into this disk.
-      * Run QEMU, with 4+G RAM, optionally SMP (multicore), both drives (in the
-        order introduced above), a NIC with model E1000
+        * Header magic: ``LBPAYLD1`` (8 bytes).
+        * Then: little-endian ``u32`` file count.
+        * Repeated for each file: little-endian ``u32`` name length,
+          little-endian ``u32`` file size, raw file name bytes, raw file bytes.
+
+      * If you are not in that mode, the second disk can still be used as an
+        optional ext3 distfiles disk, as before.
+      * Run QEMU, with 4+G RAM, optionally SMP (multicore), both drives (main
+        builder image plus payload/ext3 image), a NIC with model E1000
         (``-nic user,model=e1000``), and ``-machine kernel-irqchip=split``.
    c. **Bare metal:** Follow the same steps as QEMU, but the disks need to be
       two different *physical* disks, and boot from the first disk.
 
+Manual ``payload.img`` preparation
+----------------------------------
+
+The following script creates a raw ``payload.img`` from a manually prepared
+file list. This is equivalent to what ``rootfs.py`` does for kernel-bootstrap
+offline mode.
+
+1. Prepare a ``payload.list`` with one file per line, formatted as:
+   ``<archive-name> <absolute-path-to-archive>``.
+2. Run:
+
+   ::
+
+      cat > make-payload.sh <<'EOF'
+      #!/bin/sh
+      set -e
+      out="${1:-payload.img}"
+      list="${2:-payload.list}"
+
+      write_u32le() {
+          v="$1"
+          printf '%08x' "$v" | sed -E 's/(..)(..)(..)(..)/\4\3\2\1/' | xxd -r -p
+      }
+
+      count="$(wc -l < "${list}" | tr -d ' ')"
+      : > "${out}"
+      printf 'LBPAYLD1' >> "${out}"
+      write_u32le "${count}" >> "${out}"
+
+      while read -r name path; do
+          [ -n "${name}" ] || continue
+          size="$(wc -c < "${path}" | tr -d ' ')"
+          write_u32le "${#name}" >> "${out}"
+          write_u32le "${size}" >> "${out}"
+          printf '%s' "${name}" >> "${out}"
+          cat "${path}" >> "${out}"
+      done < "${list}"
+      EOF
+      chmod +x make-payload.sh
+      ./make-payload.sh payload.img payload.list
+
+3. Attach ``payload.img`` as an additional raw disk when booting in QEMU, or
+   as the second physical disk on bare metal.
+
+Notes:
+
+* ``payload.img`` is used in kernel-bootstrap offline mode regardless of
+  ``--build-guix-also``. With ``--build-guix-also``, the payload content is
+  larger because it also includes post-early sources from ``steps-guix``.
+* The runtime importer identifies the correct disk by checking the magic
+  ``LBPAYLD1`` on each detected block device, not by assuming a device name.
+
 Mirrors
 -------
 
diff --git a/lib/generator.py b/lib/generator.py
index 7936b4e5..964584ce 100755
--- a/lib/generator.py
+++ b/lib/generator.py
@@ -37,10 +37,14 @@ class Generator():
         self.repo_path = repo_path
         self.mirrors = mirrors
         self.build_guix_also = build_guix_also
-        self.source_manifest = self.get_source_manifest(not self.external_sources,
-                                                        build_guix_also=self.build_guix_also)
-        self.early_source_manifest = self.get_source_manifest(True,
-                                                              build_guix_also=self.build_guix_also)
+        self.source_manifest = self.get_source_manifest(
+            stop_before_improve=("get_network" if not self.external_sources else None),
+            build_guix_also=self.build_guix_also
+        )
+        self.early_source_manifest = self.get_source_manifest(
+            stop_before_improve="get_network",
+            build_guix_also=self.build_guix_also
+        )
         self.bootstrap_source_manifest = self.source_manifest
         self.payload_source_manifest = []
         self.payload_image = None
@@ -59,11 +63,17 @@ class Generator():
         """
         Split early source payload from full offline payload.
         """
-        # Keep the early builder payload small enough to avoid overrunning
-        # builder-hex0 memory file allocation before we can jump into Fiwix.
-        self.bootstrap_source_manifest = self.get_source_manifest(True, build_guix_also=False)
+        # Keep the early builder payload small: include only sources needed
+        # before improve: import_payload runs, so payload.img is the primary
+        # carrier for the rest of the offline distfiles.
+        self.bootstrap_source_manifest = self.get_source_manifest(
+            stop_before_improve="import_payload",
+            build_guix_also=False
+        )
 
-        full_manifest = self.get_source_manifest(False, build_guix_also=self.build_guix_also)
+        full_manifest = self.get_source_manifest(build_guix_also=self.build_guix_also)
+        if self.bootstrap_source_manifest == full_manifest:
+            raise ValueError("steps/manifest must include `improve: import_payload` in kernel-bootstrap mode.")
         bootstrap_set = set(self.bootstrap_source_manifest)
         self.payload_source_manifest = [entry for entry in full_manifest if entry not in bootstrap_set]
 
@@ -83,11 +93,12 @@ class Generator():
             self.check_file(distfile_path, checksum)
 
     def _create_raw_payload_image(self, target_path, manifest):
-        if not manifest:
-            return None
+        if manifest is None:
+            manifest = []
 
-        # Guarantee all payload distfiles exist and match checksums.
-        self._ensure_manifest_distfiles(manifest)
+        if manifest:
+            # Guarantee all payload distfiles exist and match checksums.
+            self._ensure_manifest_distfiles(manifest)
 
         files_by_name = {}
         for checksum, _, _, file_name in manifest:
@@ -180,7 +191,9 @@ class Generator():
             if self.repo_path or self.external_sources:
                 mkfs_args = ['-d', os.path.join(target.path, 'external')]
                 target.add_disk("external", filesystem="ext3", mkfs_args=mkfs_args)
-            elif self.payload_source_manifest:
+            else:
+                # Offline kernel-bootstrap mode keeps the early image small and
+                # puts remaining distfiles in payload.img.
                 self.payload_image = self._create_raw_payload_image(target.path, self.payload_source_manifest)
                 target.add_existing_disk("payload", self.payload_image)
         elif using_kernel:
@@ -420,7 +433,7 @@ this script the next time")
             self.check_file(path, line[0])
 
     @classmethod
-    def get_source_manifest(cls, pre_network=False, build_guix_also=False):
+    def get_source_manifest(cls, stop_before_improve=None, build_guix_also=False):
         """
         Generate a source manifest for the system.
         """
@@ -443,10 +456,13 @@ this script the next time")
 
             with open(manifest_path, 'r', encoding="utf_8") as file:
                 for line in file:
-                    if pre_network and line.strip().startswith("improve: ") and "network" in line:
-                        break
+                    stripped = line.strip()
+                    if stop_before_improve and stripped.startswith("improve: "):
+                        improve_step = stripped.split(" ")[1].split("#")[0].strip()
+                        if improve_step == stop_before_improve:
+                            break
 
-                    if not line.strip().startswith("build: "):
+                    if not stripped.startswith("build: "):
                         continue
 
                     step = line.split(" ")[1].split("#")[0].strip()
diff --git a/rootfs.py b/rootfs.py
index e0d054da..84997dff 100755
--- a/rootfs.py
+++ b/rootfs.py
@@ -71,12 +71,14 @@ with open(config_path, "r", encoding="utf-8") as cfg:
         if not line.startswith("BUILD_GUIX_ALSO=")
         and not line.startswith("MIRRORS=")
         and not line.startswith("MIRRORS_LEN=")
+        and not line.startswith("PAYLOAD_REQUIRED=")
     ]
 if build_guix_also:
     lines.append("BUILD_GUIX_ALSO=True\\n")
 if mirrors:
     lines.append(f'MIRRORS="{" ".join(mirrors)}"\\n')
     lines.append(f"MIRRORS_LEN={len(mirrors)}\\n")
+lines.append("PAYLOAD_REQUIRED=False\\n")
 with open(config_path, "w", encoding="utf-8") as cfg:
     cfg.writelines(lines)
 
@@ -470,9 +472,10 @@ print(shutil.which('chroot'))
                 arg_list += [
                     '-drive', 'file=' + target.get_disk("external") + ',format=raw',
                 ]
-            if target.get_disk("payload") is not None:
+            payload_disk = target.get_disk("payload")
+            if payload_disk is not None:
                 arg_list += [
-                    '-drive', 'file=' + target.get_disk("payload") + ',format=raw',
+                    '-drive', 'file=' + payload_disk + ',format=raw',
                 ]
             arg_list += [
                 '-machine', 'kernel-irqchip=split',
diff --git a/steps/improve/import_payload.sh b/steps/improve/import_payload.sh
index 1a8acad4..7a4b6c7b 100644
--- a/steps/improve/import_payload.sh
+++ b/steps/improve/import_payload.sh
@@ -7,5 +7,31 @@ set -ex
 
 if [ "${PAYLOAD_REQUIRED}" = True ]; then
     mkdir -p /external/distfiles
-    payload-import /external/distfiles
+    found_payload=0
+
+    # Probe all block devices reported by the running kernel instead of
+    # assuming fixed names like /dev/sdb or /dev/hdb.
+    while read -r major minor blocks name; do
+        case "${name}" in
+            ""|name|ram*|loop*|fd*|sr*|md*)
+                continue
+                ;;
+        esac
+
+        dev_path="/dev/${name}"
+        if [ ! -b "${dev_path}" ]; then
+            mknod -m 600 "${dev_path}" b "${major}" "${minor}" || :
+        fi
+
+        if payload-import --probe "${dev_path}"; then
+            payload-import --device "${dev_path}" /external/distfiles
+            found_payload=1
+            break
+        fi
+    done < /proc/partitions
+
+    if [ "${found_payload}" != 1 ]; then
+        echo "payload-import failed: no payload image found on detected block devices." >&2
+        exit 1
+    fi
 fi