Numerous computers include the ability to boot from an external hard drive. Microsoft Windows-based computers use the Master Boot Record, or MBR, partitioning scheme to separate hard drives into distinct data areas. If the MBR portion of the hard drive becomes corrupt, the computer cannot boot to that hard drive. You may receive an error that the drive is inaccessible or unmountable. Boot with the Windows installation disc and perform a few specific commands to fix an external hard drive with a corrupt MBR.
Connect the external USB hard drive to the computer.
Insert the Windows installation disc into the disc drive, then restart the computer.
Boot from the CD. Press a hot key to select the CD drive. Common hot keys are Esc, F8, F10 and F12.
Click "Repair Your Computer."
Click "Command Prompt."
Type the drive letter of the external device, followed by a colon, into the prompt. Press Enter. For example, type "E:" without quotes, then press Enter.
Type "bootrec /fixboot" without quotes, then press Enter.
Enter "bootrec /fixmbr" without quotes. Press the Enter key.
Restart the computer.