diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 92d912b..815c76c 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -30,6 +30,7 @@
 import zipfile
 
 import blockimgdiff
+from rangelib import *
 
 try:
   from hashlib import sha1 as sha1
@@ -1023,10 +1024,11 @@
 
 
 class BlockDifference:
-  def __init__(self, partition, tgt, src=None):
+  def __init__(self, partition, tgt, src=None, check_first_block=False):
     self.tgt = tgt
     self.src = src
     self.partition = partition
+    self.check_first_block = check_first_block
 
     b = blockimgdiff.BlockImageDiff(tgt, src, threads=OPTIONS.worker_threads)
     tmpdir = tempfile.mkdtemp()
@@ -1043,6 +1045,9 @@
       self._WriteUpdate(script, output_zip)
 
     else:
+      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()))
@@ -1072,6 +1077,18 @@
             (self.device, partition, partition, partition))
     script.AppendExtra(script._WordWrap(call))
 
+  def _CheckFirstBlock(self, script):
+    r = RangeSet((0, 1))
+    h = sha1()
+    for data in self.src.ReadRangeSet(r):
+      h.update(data)
+    h = h.hexdigest()
+
+    script.AppendExtra(('(range_sha1("%s", "%s") == "%s") || '
+                        'abort("%s has been remounted R/W; '
+                        'reflash device to reenable OTA updates");')
+                       % (self.device, r.to_string_raw(), h, self.device))
+
 
 DataImage = blockimgdiff.DataImage
 
diff --git a/tools/releasetools/ota_from_target_files b/tools/releasetools/ota_from_target_files
index 8b7342b..6f34450 100755
--- a/tools/releasetools/ota_from_target_files
+++ b/tools/releasetools/ota_from_target_files
@@ -731,14 +731,16 @@
 
   system_src = GetImage("system", OPTIONS.source_tmp, OPTIONS.source_info_dict)
   system_tgt = GetImage("system", OPTIONS.target_tmp, OPTIONS.target_info_dict)
-  system_diff = common.BlockDifference("system", system_tgt, system_src)
+  system_diff = common.BlockDifference("system", system_tgt, system_src,
+                                       check_first_block=True)
 
   if HasVendorPartition(target_zip):
     if not HasVendorPartition(source_zip):
       raise RuntimeError("can't generate incremental that adds /vendor")
     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)
+    vendor_diff = common.BlockDifference("vendor", vendor_tgt, vendor_src,
+                                         check_first_block=True)
   else:
     vendor_diff = None