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Usually, during installing Windows 7, the installer would create an extra system reserved partition for storing Windows Boot Manager files.

If you backup Windows 7 from a computer and restore it to another computer without backing up system reserved partition,
you will lose Windows Boot Manager files. The steps to restore Windows Boot Manager are:
1. Boot from Windows 7 installation CD.
2. After the Windows 7 installer GUI shows, pressing Shift+F10 to invoke a command window.
3. Execute this command (modify this command as your needs):
bcdboot C:\Windows /s C:
4. Close the command window and the Windows 7 installer GUI.

Attentions! Don't use a force shutdown at step 4,
please certainly let Windows 7 installer shutdown your computer.
Otherwise, the data of your Windows partition would corrupt!