diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 544b1d0..0a387ec 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -82,6 +82,7 @@
   """A zero-length image."""
   blocksize = 4096
   care_map = RangeSet()
+  clobbered_blocks = RangeSet()
   total_blocks = 0
   file_map = {}
   def ReadRangeSet(self, ranges):
@@ -114,6 +115,7 @@
 
     self.total_blocks = len(self.data) / self.blocksize
     self.care_map = RangeSet(data=(0, self.total_blocks))
+    self.clobbered_blocks = RangeSet()
 
     zero_blocks = []
     nonzero_blocks = []
@@ -135,6 +137,8 @@
     return [self.data[s*self.blocksize:e*self.blocksize] for (s, e) in ranges]
 
   def TotalSha1(self):
+    # DataImage always carries empty clobbered_blocks.
+    assert self.clobbered_blocks.size() == 0
     return sha1(self.data).hexdigest()
 
 
@@ -184,6 +188,10 @@
 #      (Typically a domain is a file, and the key in file_map is the
 #      pathname.)
 #
+#    clobbered_blocks: a RangeSet containing which blocks contain data
+#      but may be altered by the FS. They need to be excluded when
+#      verifying the partition integrity.
+#
 #    ReadRangeSet(): a function that takes a RangeSet and returns the
 #      data contained in the image blocks of that RangeSet.  The data
 #      is returned as a list or tuple of strings; concatenating the
@@ -193,7 +201,7 @@
 #
 #    TotalSha1(): a function that returns (as a hex string) the SHA-1
 #      hash of all the data in the image (ie, all the blocks in the
-#      care_map)
+#      care_map minus clobbered_blocks).
 #
 # When creating a BlockImageDiff, the src image may be None, in which
 # case the list of transfers produced will never read from the
@@ -844,6 +852,12 @@
                  "zero", self.transfers)
         continue
 
+      elif tgt_fn == "__COPY":
+        # "__COPY" domain includes all the blocks not contained in any
+        # file and that need to be copied unconditionally to the target.
+        Transfer(tgt_fn, None, tgt_ranges, empty, "new", self.transfers)
+        continue
+
       elif tgt_fn in self.src.file_map:
         # Look for an exact pathname match in the source.
         Transfer(tgt_fn, tgt_fn, tgt_ranges, self.src.file_map[tgt_fn],
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index ab58fa7..8ab98e7 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -1148,6 +1148,9 @@
     self.partition = partition
     self.check_first_block = check_first_block
 
+    # Due to http://b/20939131, check_first_block is disabled temporarily.
+    assert not self.check_first_block
+
     if version is None:
       version = 1
       if OPTIONS.info_dict:
@@ -1181,18 +1184,18 @@
     if not self.src:
       script.Print("Image %s will be patched unconditionally." % (partition,))
     else:
+      ranges = self.src.care_map.subtract(self.src.clobbered_blocks)
+      ranges_str = ranges.to_string_raw()
       if self.version >= 3:
         script.AppendExtra(('if (range_sha1("%s", "%s") == "%s" || '
                             'block_image_verify("%s", '
                             'package_extract_file("%s.transfer.list"), '
                             '"%s.new.dat", "%s.patch.dat")) then') % (
-                            self.device, self.src.care_map.to_string_raw(),
-                            self.src.TotalSha1(),
+                            self.device, ranges_str, self.src.TotalSha1(),
                             self.device, partition, partition, partition))
       else:
         script.AppendExtra('if range_sha1("%s", "%s") == "%s" then' % (
-            self.device, self.src.care_map.to_string_raw(),
-            self.src.TotalSha1()))
+                           self.device, ranges_str, self.src.TotalSha1()))
       script.Print('Verified %s image...' % (partition,))
       script.AppendExtra('else')
 
@@ -1240,6 +1243,9 @@
 
     return ctx.hexdigest()
 
+  # TODO(tbao): Due to http://b/20939131, block 0 may be changed without
+  # remounting R/W. Will change the checking to a finer-grained way to
+  # mask off those bits.
   def _CheckFirstBlock(self, script):
     r = rangelib.RangeSet((0, 1))
     srchash = self._HashBlocks(self.src, r)
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index 900eaec..c4d0c1b 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -475,7 +475,13 @@
       path = add_img_to_target_files.BuildVendor(
           tmpdir, info_dict, block_list=mappath)
 
-  return sparse_img.SparseImage(path, mappath)
+  # Bug: http://b/20939131
+  # In ext4 filesystems, block 0 might be changed even being mounted
+  # R/O. We add it to clobbered_blocks so that it will be written to the
+  # target unconditionally. Note that they are still part of care_map.
+  clobbered_blocks = "0"
+
+  return sparse_img.SparseImage(path, mappath, clobbered_blocks)
 
 
 def WriteFullOTAPackage(input_zip, output_zip):
@@ -773,7 +779,6 @@
         OPTIONS.info_dict.get("blockimgdiff_versions", "1").split(","))
 
   system_diff = common.BlockDifference("system", system_tgt, system_src,
-                                       check_first_block=True,
                                        version=blockimgdiff_version)
 
   if HasVendorPartition(target_zip):
@@ -784,7 +789,6 @@
     vendor_tgt = GetImage("vendor", OPTIONS.target_tmp,
                           OPTIONS.target_info_dict)
     vendor_diff = common.BlockDifference("vendor", vendor_tgt, vendor_src,
-                                         check_first_block=True,
                                          version=blockimgdiff_version)
   else:
     vendor_diff = None
diff --git a/tools/releasetools/sparse_img.py b/tools/releasetools/sparse_img.py
index b97bb84..2ac97ac 100644
--- a/tools/releasetools/sparse_img.py
+++ b/tools/releasetools/sparse_img.py
@@ -21,10 +21,17 @@
 
 
 class SparseImage(object):
-  """Wraps a sparse image file (and optional file map) into an image
-  object suitable for passing to BlockImageDiff."""
+  """Wraps a sparse image file into an image object.
 
-  def __init__(self, simg_fn, file_map_fn=None):
+  Wraps a sparse image file (and optional file map and clobbered_blocks) into
+  an image object suitable for passing to BlockImageDiff. file_map contains
+  the mapping between files and their blocks. clobbered_blocks contains the set
+  of blocks that should be always written to the target regardless of the old
+  contents (i.e. copying instead of patching). clobbered_blocks should be in
+  the form of a string like "0" or "0 1-5 8".
+  """
+
+  def __init__(self, simg_fn, file_map_fn=None, clobbered_blocks=None):
     self.simg_f = f = open(simg_fn, "rb")
 
     header_bin = f.read(28)
@@ -57,6 +64,7 @@
     pos = 0   # in blocks
     care_data = []
     self.offset_map = offset_map = []
+    self.clobbered_blocks = rangelib.RangeSet(data=clobbered_blocks)
 
     for i in range(total_chunks):
       header_bin = f.read(12)
@@ -103,7 +111,7 @@
     self.offset_index = [i[0] for i in offset_map]
 
     if file_map_fn:
-      self.LoadFileBlockMap(file_map_fn)
+      self.LoadFileBlockMap(file_map_fn, self.clobbered_blocks)
     else:
       self.file_map = {"__DATA": self.care_map}
 
@@ -111,9 +119,10 @@
     return [d for d in self._GetRangeData(ranges)]
 
   def TotalSha1(self):
-    """Return the SHA-1 hash of all data in the 'care' regions of this image."""
+    """Return the SHA-1 hash of all data in the 'care' regions but not in
+    clobbered_blocks of this image."""
     h = sha1()
-    for d in self._GetRangeData(self.care_map):
+    for d in self._GetRangeData(self.care_map.subtract(self.clobbered_blocks)):
       h.update(d)
     return h.hexdigest()
 
@@ -156,7 +165,7 @@
           yield fill_data * (this_read * (self.blocksize >> 2))
         to_read -= this_read
 
-  def LoadFileBlockMap(self, fn):
+  def LoadFileBlockMap(self, fn, clobbered_blocks):
     remaining = self.care_map
     self.file_map = out = {}
 
@@ -166,14 +175,20 @@
         ranges = rangelib.RangeSet.parse(ranges)
         out[fn] = ranges
         assert ranges.size() == ranges.intersect(remaining).size()
+
+        # Currently we assume that blocks in clobbered_blocks are not part of
+        # any file.
+        assert not clobbered_blocks.overlaps(ranges)
         remaining = remaining.subtract(ranges)
 
+    remaining = remaining.subtract(clobbered_blocks)
+
     # For all the remaining blocks in the care_map (ie, those that
-    # aren't part of the data for any file), divide them into blocks
-    # that are all zero and blocks that aren't.  (Zero blocks are
-    # handled specially because (1) there are usually a lot of them
-    # and (2) bsdiff handles files with long sequences of repeated
-    # bytes especially poorly.)
+    # aren't part of the data for any file nor part of the clobbered_blocks),
+    # divide them into blocks that are all zero and blocks that aren't.
+    # (Zero blocks are handled specially because (1) there are usually
+    # a lot of them and (2) bsdiff handles files with long sequences of
+    # repeated bytes especially poorly.)
 
     zero_blocks = []
     nonzero_blocks = []
@@ -203,6 +218,7 @@
 
     out["__ZERO"] = rangelib.RangeSet(data=zero_blocks)
     out["__NONZERO"] = rangelib.RangeSet(data=nonzero_blocks)
+    out["__COPY"] = clobbered_blocks
 
   def ResetFileMap(self):
     """Throw away the file map and treat the entire image as