How to Fix an MBR Inaccessible External Hard Drive

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.

External hard drive for backup.
How to Fix an MBR Inaccessible External Hard Drive
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Step

Connect the external USB hard drive to the computer.

Step

Insert the Windows installation disc into the disc drive, then restart the computer.

Step

Boot from the CD. Press a hot key to select the CD drive. Common hot keys are Esc, F8, F10 and F12.

Step

Click "Repair Your Computer."

Step

Click "Command Prompt."

Step

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.

Step

Type "bootrec /fixboot" without quotes, then press Enter.

Step

Enter "bootrec /fixmbr" without quotes. Press the Enter key.

Step

Restart the computer.