I have an external USB hard drive with a defect in master boot record. When I plug in the USB HD, my computer detects it and assigns a letter (D to it. However, I cannot format it, as I got an error message related to I/O error. I know it is a master boot record error, because when I used another software (couldn't remember the name), it says that the disk has a master boot record error. Question: 1) Is there a free software that can repair the external usb hard drive? The software must be able to run on Windows 7 64 bit OS.