diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 8b179d5..51031ce 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -73,6 +73,7 @@
   """A zero-length image."""
   blocksize = 4096
   care_map = RangeSet()
+  clobbered_blocks = RangeSet()
   total_blocks = 0
   file_map = {}
   def ReadRangeSet(self, ranges):
@@ -105,6 +106,7 @@
 
     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 = []
@@ -126,9 +128,9 @@
     return [self.data[s*self.blocksize:e*self.blocksize] for (s, e) in ranges]
 
   def TotalSha1(self):
-    if not hasattr(self, "sha1"):
-      self.sha1 = sha1(self.data).hexdigest()
-    return self.sha1
+    # DataImage always carries empty clobbered_blocks.
+    assert self.clobbered_blocks.size() == 0
+    return sha1(self.data).hexdigest()
 
 
 class Transfer(object):
@@ -174,6 +176,10 @@
 #      (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
@@ -183,7 +189,7 @@
 #
 #    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
@@ -768,6 +774,13 @@
                  "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.
+        print("FindTransfers: new", tgt_ranges);
+        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 a596c26..b28cbf1 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -1030,6 +1030,9 @@
     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:
@@ -1058,29 +1061,28 @@
     self._WriteUpdate(script, output_zip)
 
   def WriteVerifyScript(self, script):
+    partition = self.partition
     if not self.src:
-      script.Print("Image %s will be patched unconditionally." % (self.partition,))
+      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()
+      script.AppendExtra('if range_sha1("%s", "%s") == "%s" then' % (
+                         self.device, ranges_str, self.src.TotalSha1()))
+      script.Print('Verified %s image...' % (partition,))
+      script.AppendExtra('else')
+
+      # When generating incrementals for the system and vendor partitions,
+      # explicitly check the first block (which contains the superblock) of
+      # the partition to see if it's what we expect. If this check fails,
+      # give an explicit log message about the partition having been
+      # remounted R/W (the most likely explanation) and the need to flash to
+      # get OTAs working again.
       if self.check_first_block:
         self._CheckFirstBlock(script)
 
-      script.AppendExtra('if range_sha1("%s", "%s") == "%s" then' %
-                         (self.device, self.src.care_map.to_string_raw(),
-                          self.src.TotalSha1()))
-      script.Print("Verified %s image..." % (self.partition,))
-      # Abort the OTA update if it doesn't support resumable OTA (i.e. version<3)
-      # and the checksum doesn't match the one in the source partition.
-      if self.version < 3:
-        script.AppendExtra(('else\n'
-                            '  abort("%s partition has unexpected contents");\n'
-                            'endif;') % (self.partition))
-      else:
-        script.AppendExtra(('else\n'
-                            '  (range_sha1("%s", "%s") == "%s") ||\n'
-                            '  abort("%s partition has unexpected contents");\n'
-                            'endif;') %
-                           (self.device, self.tgt.care_map.to_string_raw(),
-                            self.tgt.TotalSha1(), self.partition))
+      script.AppendExtra(('  abort("%s partition has unexpected contents");\n'
+                          'endif;') % (partition))
 
   def _WriteUpdate(self, script, output_zip):
     partition = self.partition
@@ -1098,6 +1100,9 @@
             (self.device, partition, partition, partition))
     script.AppendExtra(script._WordWrap(call))
 
+  # 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 = RangeSet((0, 1))
     h = sha1()
diff --git a/tools/releasetools/ota_from_target_files b/tools/releasetools/ota_from_target_files
index 25309a4..768f4cb 100755
--- a/tools/releasetools/ota_from_target_files
+++ b/tools/releasetools/ota_from_target_files
@@ -457,7 +457,13 @@
       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):
@@ -748,7 +754,7 @@
         OPTIONS.info_dict.get("blockimgdiff_versions", "1").split(","))
 
   system_diff = common.BlockDifference("system", system_tgt, system_src,
-                                       check_first_block=True,
+                                       check_first_block=False,
                                        version=blockimgdiff_version)
 
   if HasVendorPartition(target_zip):
@@ -757,7 +763,7 @@
     vendor_src = GetImage("vendor", OPTIONS.source_tmp, OPTIONS.source_info_dict)
     vendor_tgt = GetImage("vendor", OPTIONS.target_tmp, OPTIONS.target_info_dict)
     vendor_diff = common.BlockDifference("vendor", vendor_tgt, vendor_src,
-                                         check_first_block=True,
+                                         check_first_block=False,
                                          version=blockimgdiff_version)
   else:
     vendor_diff = None
diff --git a/tools/releasetools/sparse_img.py b/tools/releasetools/sparse_img.py
index 7574747..e253fbf 100644
--- a/tools/releasetools/sparse_img.py
+++ b/tools/releasetools/sparse_img.py
@@ -22,10 +22,17 @@
 from rangelib import *
 
 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)
@@ -59,6 +66,7 @@
     pos = 0   # in blocks
     care_data = []
     self.offset_map = offset_map = []
+    self.clobbered_blocks = RangeSet(data=clobbered_blocks)
 
     for i in range(total_chunks):
       header_bin = f.read(12)
@@ -106,7 +114,7 @@
     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}
 
@@ -114,9 +122,10 @@
     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()
 
@@ -159,7 +168,7 @@
           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 = {}
 
@@ -169,14 +178,20 @@
         ranges = 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 = []
@@ -206,6 +221,7 @@
 
     out["__ZERO"] = RangeSet(data=zero_blocks)
     out["__NONZERO"] = RangeSet(data=nonzero_blocks)
+    out["__COPY"] = clobbered_blocks
 
   def ResetFileMap(self):
     """Throw away the file map and treat the entire image as