diff --git a/lib/cros_image_common.sh b/lib/cros_image_common.sh
index 7c9befcc17..c4732c221c 100644
--- a/lib/cros_image_common.sh
+++ b/lib/cros_image_common.sh
@@ -92,15 +92,13 @@ image_dump_partial_file() {
   local sectors="$3"
   local bs=512
 
-  # Try to use larger buffer if offset/size can be re-aligned.
-  # 2M / 512 = 4096
-  local buffer_ratio=4096
-  if [ $((offset % buffer_ratio)) -eq 0 -a \
-       $((sectors % buffer_ratio)) -eq 0 ]; then
-    offset=$((offset / buffer_ratio))
-    sectors=$((sectors / buffer_ratio))
-    bs=$((bs * buffer_ratio))
-  fi
+  # Increase buffer size as much as possible until 8M
+  while [ $((bs < (8 * 1024 * 1024) && sectors > 0 &&
+             offset % 2 == 0 && sectors % 2 == 0)) = "1" ]; do
+    bs=$((bs * 2))
+    offset=$((offset / 2))
+    sectors=$((sectors / 2))
+  done
 
   if image_has_command pv; then
     dd if="$file" bs=$bs skip="$offset" count="$sectors" \
@@ -124,6 +122,43 @@ image_dump_partition() {
   image_dump_partial_file "$file" "$offset" "$size"
 }
 
+# Updates a file (from stdin) by given offset and size (in sectors)
+image_update_partial_file() {
+  local file="$1"
+  local offset="$2"
+  local sectors="$3"
+  local bs=512
+
+  # Increase buffer size as much as possible until 8M
+  while [ $((bs < (8 * 1024 * 1024) && sectors > 0 &&
+             offset % 2 == 0 && sectors % 2 == 0)) = "1" ]; do
+    bs=$((bs * 2))
+    offset=$((offset / 2))
+    sectors=$((sectors / 2))
+  done
+
+  if image_has_command pv; then
+    pv -ptreb -B $bs -s $((sectors * bs)) |
+      dd of="$file" bs=$bs seek="$offset" count="$sectors" \
+        iflag=fullblock oflag=dsync conv=notrunc status=noxfer 2>/dev/null
+  else
+    dd of="$file" bs=$bs seek="$offset" count="$sectors" \
+      iflag=fullblock oflag=dsync conv=notrunc status=noxfer 2>/dev/null
+  fi
+}
+
+# Updates a specific partition in given image file (from stdin)
+image_update_partition() {
+  local file="$1"
+  local part_num="$2"
+  local offset="$(image_part_offset "$file" "$part_num")" ||
+    image_die "failed to find partition #$part_num from: $file"
+  local size="$(image_part_size "$file" "$part_num")" ||
+    image_die "failed to find partition #$part_num from: $file"
+
+  image_update_partial_file "$file" "$offset" "$size"
+}
+
 # Maps a specific partition from given image file to a loop device
 image_map_partition() {
   local file="$1"
@@ -173,34 +208,39 @@ image_umount_partition() {
   sudo umount -d "$mount_point"
 }
 
-# Copy a partition from one image to another.
+# Copy a partition from one image to another (size must be equal)
 image_partition_copy() {
-  local src="$1"
-  local srcpart="$2"
-  local dst="$3"
-  local dstpart="$4"
-
-  local srcoffset=$(image_part_offset "${src}" "${srcpart}")
-  local dstoffset=$(image_part_offset "${dst}" "${dstpart}")
-  local length=$(image_part_size "${src}" "${srcpart}")
-  local dstlength=$(image_part_size "${dst}" "${dstpart}")
-
-  if [ "${length}" -gt  "${dstlength}" ]; then
-    exit 1
+  local src="$1" src_part="$2" dst="$3" dst_part="$4"
+  local size1="$(image_part_size "$src" "$src_part")"
+  local size2="$(image_part_size "$dst" "$dst_part")"
+  if [ "$size1" != "$size2" ]; then
+    die "Partition size different: ($size1 != $size2)"
   fi
+  image_dump_partition "$src" "$src_part" 2>/dev/null |
+    image_update_partition "$dst" "$dst_part"
+}
 
-  # Try to use larger buffer if offset/size can be re-aligned.
-  # 2M / 512 = 4096
-  local buffer_ratio=4096
-  local bs=512
-  if [ $((dstoffset % buffer_ratio)) -eq 0 -a \
-       $((length % buffer_ratio)) -eq 0 ]; then
-    dstoffset=$((dstoffset / buffer_ratio))
-    bs=$((bs * buffer_ratio))
+# Copy a partition from one image to another (source <= dest)
+image_partition_overwrite() {
+  local src="$1" src_part="$2" dst="$3" dst_part="$4"
+  local size1="$(image_part_size "$src" "$src_part")"
+  local size2="$(image_part_size "$dst" "$dst_part")"
+  if [ "$size1" -gt "$size2" ]; then
+    die "Destination is too small: ($size1 > $size2)"
   fi
+  image_dump_partition "$src" "$src_part" 2>/dev/null |
+    image_update_partition "$dst" "$dst_part"
+}
 
-  image_dump_partition "${src}" "${srcpart}" |
-      dd of="${dst}" bs="${bs}" seek="${dstoffset}" conv=notrunc oflag=dsync
+# Copy a partition image from file to a full disk image.
+image_partition_copy_from_file() {
+  local src="$1" dst="$2" dst_part="$3"
+  local size1="$(($(stat -c"%s" "$src") / 512))"
+  local size2="$(image_part_size "$dst" "$dst_part")"
+  if [ "$size1" != "$size2" ]; then
+    die "Partition size different: ($size1 != $size2)"
+  fi
+  cat "$src" | image_update_partition "$dst" "$dst_part"
 }
 
 # Temporary object management
diff --git a/make_factory_package.sh b/make_factory_package.sh
index dfeeb5c7d8..88a2e930ec 100755
--- a/make_factory_package.sh
+++ b/make_factory_package.sh
@@ -230,8 +230,8 @@ prepare_img() {
         "$(stat -c %s ${outdev})" != "$(( ${sectors} * 512 ))"  -o \
         "$FLAGS_preserve" = "$FLAGS_FALSE" ]; then
     echo "Generating empty image file"
-    image_dump_partial_file /dev/zero 0 "${sectors}" |
-        dd of="${outdev}" bs=8M
+    truncate -s "0" "$outdev"
+    truncate -s "$((sectors * 512))" "$outdev"
   else
     echo "Reusing $outdev"
   fi
@@ -381,28 +381,23 @@ generate_img() {
   local release_image="${RELEASE_DIR}/${RELEASE_IMAGE}"
   echo "Release Kernel"
   if [ -n "$RELEASE_KERNEL" ]; then
-    local newkernel="$(image_map_partition "${outdev}" "4")"
-    local failed=""
-    sudo dd if="$RELEASE_KERNEL" of="$newkernel" bs=512 ||
-      failed="TRUE"
-    image_unmap_partition "$newkernel"
-    [ -z "$failed" ] || die "Failed to build release kernel."
+    image_partition_copy_from_file "${RELEASE_KERNEL}" "${outdev}" 4
   else
     image_partition_copy "${release_image}" 2 "${outdev}" 4
   fi
   echo "Release Rootfs"
-  image_partition_copy "${release_image}" 3 "${outdev}" 5
+  image_partition_overwrite "${release_image}" 3 "${outdev}" 5
   echo "OEM parition"
-  image_partition_copy "${release_image}" 8 "${outdev}" 8
+  image_partition_overwrite "${release_image}" 8 "${outdev}" 8
 
   # Go to retrieve the factory test image.
   local factory_image="${FACTORY_DIR}/${FACTORY_IMAGE}"
   echo "Factory Kernel"
   image_partition_copy "${factory_image}" 2 "${outdev}" 2
   echo "Factory Rootfs"
-  image_partition_copy "${factory_image}" 3 "${outdev}" 3
+  image_partition_overwrite "${factory_image}" 3 "${outdev}" 3
   echo "Factory Stateful"
-  image_partition_copy "${factory_image}" 1 "${outdev}" 1
+  image_partition_overwrite "${factory_image}" 1 "${outdev}" 1
   echo "EFI Partition"
   image_partition_copy "${factory_image}" 12 "${outdev}" 12
   apply_hwid_updater "${hwid_updater}" "${outdev}"