Blasting MBRs

Installing and removing versions of Windows NT, Windows 9x, and Linux on a computer often leads to mysterious computer behavior. Recently, I thought that I had completely removed Linux from a computer, but the computer wouldn't boot without a Linux-ish error message. I zapped every partition and still Linux raised its head—despite a seemingly empty hard disk. PC veterans attribute mysteries of this type to an area of the disk called the Master Boot Record (MBR). Unless you know a few tricks, NT, other versions of Windows, and DOS lack simple tools for working with the MBR.

The MBR sits on your hard disk's first sector in a 512-byte area. The MBR provides a table of contents of the disk's partitions and their sizes. As a result, formatting the C drive to delete the MBR would be ineffective because the MBR sits outside the area of the disk known as drive C. Think of it this way: The MBR is a book's table of contents, and the C drive is the first chapter; ripping the pages of the first chapter out of the book wouldn't remove the first chapter's entry in the table of contents. The MBR has two parts: a short program that performs part of your PC's booting process and data that represents the table of contents.

In my case of trying to remove Linux, Linux modified the program inside the MBR, and the program emitted the error message. When you first try to fix a bulky MBR, try Fdisk. Fdisk is the only NT, Windows, and DOS command that directly affects the MBR. Fdisk has an undocumented option that replaces your computer's original MBR program: Run DOS or Windows, open up a command line, and type

fdisk /mbr 

This procedure should replace the MBR, but sometimes it won't. Fdisk does a sanity check before writing out this program, and sometimes Fdisk decides not to rewrite the MBR's program. Unfortunately, that's the error message I saw when trying to use Fdisk on my Linux system. I have no idea why Fdisk decided not to rewrite the program. I've removed Linux from systems without getting back talk from Fdisk, but for some reason, Fdisk wasn't cooperating that day. I figured that the best approach was to wipe the MBR clean and rebuild the drive. But I had to figure out how to accomplish this task with Fdisk failing me. Time for Plan B and a little DEBUG magic.

Since 1983, Microsoft has shipped its OSs (i.e., NT, Windows, and DOS) with a helpful tool called DEBUG. A cool feature of DEBUG lets you create and execute small assembly language programs; let's create a program to delete the MBR's data. This assembly language program won't run under NT, so you need to boot Win9x or DOS from a safe-mode command prompt.

Before creating this program, however, let me stress a warning. This program intends to remove your hard disk's main table of contents. If you're successful, you will delete the entire hard disk. Don't try this procedure if you want to save your hard disk's data. This procedure is intended only for drives that you want to restore to their factory-shipped state.

At a command prompt, start DEBUG. You know DEBUG is running when you see a hyphen (-). The hyphen is DEBUG's way of saying, "DEBUG reporting for duty and standing by." DEBUG commands generally consist of one letter followed by letters and numbers. (By the way, the numbers will be in hexadecimal format.)