Monday, January 30, 2006

Why PCs Die - Part 2

In the last couple of months we've seen a number of Windows XP PCs that won't boot when turned on. Typically the PC appears to start OK by displaying the coloured XP splash screen but then proceeds to the text-only start up screen that gives the option to boot to Safe Mode or to the Last Known Good Configuration. No matter what is selected at this screen the PC either reboots and goes through the splash screen and finishes up again at the text-only start up screen or it results in a stop error screen, more commonly called a blue screen of death or BSOD. The information on the BSOD can vary but generally will include one of the following statements:
Invalid partition table
Error loading operating system
Missing operating system

Getting a message like this can be pretty scary and may cause you to overreact and try to reformat your hard disc and reinstall the operating system - thereby losing any data not previously backed-up.

The cause of the behaviour noted above is generally a corrupt master boot record (MBR). Every hard disk must have a consistent "starting point" where key information is stored about the disk, such as how many partitions it has, what sort of partitions they are, etc. There also needs to be somewhere that the BIOS can load the initial boot program that starts the process of loading the operating system. The place where this information is stored is the MBR and it is always located at cylinder 0, head 0, and sector 1, the first sector on the hard disk. If
the MBR becomes corrupt then the boot process will fail as described above.

Typical causes of corruption in the MBR include it being infected by malicious code (viruses), becoming corrupted by disk errors, or being overwritten by other boot loaders when experimenting with multiple operating systems on a host e.g. GRUB or LILO used by Linux.

Generally corrupt MBRs can be rebuilt using the appropriate recovery tools and, to date, none of our customers have reported a recurrence of the problem.

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