How to Repair Windows 7 from an External Hard Drive

If your Windows 7 installation is corrupted or unbootable, you may be able to repair it from an external hard drive. Here’s how:

Prepare the External Hard Drive

Connect the external hard drive to your computer.
Boot from a Windows 7 installation disc or USB drive.
Select your language, time, and keyboard input, then click “Next.”
Click “Repair your computer” in the lower-left corner.

Launch Command Prompt

Press the “Shift F10” key combination to open Command Prompt.

Repair the Master Boot Record (MBR)

In Command Prompt, type the following commands, pressing Enter after each one:
diskpart
list volume
select volume # (replace # with the number of your external hard drive’s volume)
active
exit
Type bootrec /fixmbr and press Enter.

Rebuild the Boot Configuration Data (BCD)

Type bootrec /rebuildbcd and press Enter.
Follow the prompts to add the Windows installation on your external hard drive to the BCD store.

Fix Boot Errors

Type bootrec /fixboot and press Enter.
If you encounter any other boot errors, use the bootrec utility to fix them:
bootrec /scanos (scans for other Windows installations)
bootrec /rebuildbcd (rebuilds the BCD store)
bootrec /fixboot (fixes boot sector errors)

Exit and Restart

Type exit to close Command Prompt.
Click “Restart” to boot from your external hard drive.
If the repair process is successful, you should be able to boot into your Windows 7 installation on the external hard drive. Keep in mind that performance may be slower compared to booting from an internal hard drive.