在AndroidManifest.xml中声明
android:sharedUserId="android.uid.system"
后，该应用程序将没有权限读写SD卡。
Processes that continue holding open fds on the sdcard a little after it is
requested to be unmounted will be killed so that it can unmount.
We don't want the system process to be able to access the sdcard to avoid
these kinds of issues (and just general security cleanliness), so that it
does not have permission to access it.
http://groups.google.com/group/android-porting/browse_thread/thread/c6af65d7ddd5eff3http://stackoverflow.com/questions/5617797/android-shared-user-id-and-reading-writing-a-filehttp://stackoverflow.com/questions/6679011/it-cant-access-the-sdcard-after-setting-android-uid-system-in-my-applicationhttp://www.eoeandroid.com/forum.php?mod=viewthread&tid=63314&page=1#pid636737