Try to mount new filesystem w/ ext2 first, then ext3.

postinstall_runner_action.cc

 // Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "update_engine/postinstall_runner_action.h"
 #include <sys/mount.h>
 #include <stdlib.h>
 #include <vector>
 #include "update_engine/subprocess.h"
 #include "update_engine/utils.h"
@@ skipping 14 matching lines @@
   ScopedActionCompleter completer(processor_, this);
 
   // Make mountpoint.
   TEST_AND_RETURN(utils::MakeTempDirectory("/tmp/au_postint_mount.XXXXXX",
                                            &temp_rootfs_dir_));
   ScopedDirRemover temp_dir_remover(temp_rootfs_dir_);
 
   unsigned long mountflags = MS_RDONLY;
   int rc = mount(install_device.c_str(),
                  temp_rootfs_dir_.c_str(),
-                 "ext4",
+                 "ext2",
                  mountflags,
                  NULL);
   if (rc < 0) {
-    LOG(INFO) << "Failed to mount install part as ext4. Trying ext3.";
+    LOG(INFO) << "Failed to mount install part as ext2. Trying ext3.";
     rc = mount(install_device.c_str(),
                temp_rootfs_dir_.c_str(),
                "ext3",
                mountflags,
                NULL);
   }
   if (rc < 0) {
     LOG(ERROR) << "Unable to mount destination device " << install_device
                << " onto " << temp_rootfs_dir_;
     return;
@@ skipping 25 matching lines @@
 }
 
 void PostinstallRunnerAction::StaticCompletePostinstall(int return_code,
                                                         const string& output,
                                                         void* p) {
   reinterpret_cast<PostinstallRunnerAction*>(p)->CompletePostinstall(
       return_code);
 }
 
 }  // namespace chromeos_update_engine