BleepingComputer.com: Does the BIOS Read the Master Boot Record?

I tried searching for ths information before posting but got too many hits on MBR and lack the skills to make a better search argument.

I need to know if the MBR is read by BIOS or by the OS or by both of them. If the MBR is read only by the OS, what happens if the OS (winXP Pro SP3) is booted from CD or USB thumb drive, i.e., does the OS try to read the HDD for the MBR or does it get it from the boot media? If the HDD is read, either by the BIOS or the OS, how then can one fix a corrupted or missing MBR?

I have a laptop with a new hard drive that had the OS running on it without problems until I tried to delete some files and folders recovered from back up media. The problem arose because I first tried to run Acronis file/folder recovery on a too small HDD. I ran a backup of that small HDD and recovered everything after I installed the new, larger HDD. Upon realizing what I'd done, I used Windows delete commands to try to get rid of the duplicate files before recovering files from the other Acronis backup from a second, separate USB external HDD. I was not able to boot after those attempted deletions -they never competed- either from the internal HDD, a recovery CD, the external USB Acronis image, or from a bootable USB thumb drive. Without being able to boot I couldn't get to Recoery Console to rub fixboot or any other command line processes. Every attempt to boot from whatever source produced a black screen with the error message: [XLDR] ATA error. I believe refers to a Windows NT loader error but I am only assuming that the error arises from a bad boot sector, I.e., MBR, because I cannot get past the error to rub diagnostics.

If anyone can explain this for me, I would be very grateful.

(Apologies for typos - iPads and their spell correctors are maddening)

I can't follow what you did, but to answer your questions:

BIOS doesn't "read" anything. If you boot to the bios you do so before the MBR is accessed.

If you boot to a CD or DVD, the MBR is not accessed (because the HD is not accessed)

Did that answer your questions?

Allan, on 02 December 2011 - 12:38 PM, said:

Yes. Thank you for the response.

Your answer leads me to refine my question, though.

Perhaps I didn't make it clear that I was NOT able to boot from any device. I deduced from the loader ATA error that a boot sector problem - probably a corrupted MBR, but possibly something else, caused the problem. Everything I could find about my symptoms indicated that the fixboot process should be run as the first repair step. I was unable to boot; therefore I was usable to get to Recovery Console or a command prompt. I don't have DOS on anything but floppy disks and no FDD on the laptop. None of my recovery tools (Acronis, windows backup CD, UBCD, and USB repair stick) would get me to a command prompt so I can't even reformat the HDD.

I guess I'm just stuck on stupid. I really would like to knw how to get out of the situation. Of course, I will buy another HDD and install it and reinstall the OS, but I believe the disk drive would be ok if I could recover whatever boot sector problem is at root. It is a new drive and the laptop runs great on the small capacity (120GB) HDD.

Like I said, "I'm stuck on stupid!"

This post has been edited by rabbitHutch: 02 December 2011 - 02:16 PM

If you are getting that error you are not booting to the cd or dvd. There are essentially three reasons you may not be booting to the cd:

1) The optical drive is not the first device in the bios boot order.
2) The cd is bad or not bootable
3) The cd drive is bad

The usual culprit it #1. Make sure your cd drive is the first device in the bios boot order, then make sure you are booting to an original, retail XP CD

Allan, on 02 December 2011 - 02:32 PM, said:

Thanks, again, for the response, Allan.

I would agree with you except for one fact. By simply removing and replacing the 500GB HDD with the 120 GB HDD, the system boots - no changes in BIOS settings, no changes to any other hardware, just swapping HDDs and I can boot from HDD or from CD. I've been around this ensuring that nothing changed but the HDD so many times that I'm certain te error follows the installation of the 500 GB HDD.

I've never seen anything like it before. There is something causing the HDD to be selected, probably with a read command.

Unless you can suggest other culprits.

Thans for the responses.

I can tell you for an absolute fact that the hd has absolutely nothing to do with your ability to boot to a cd. I don't know what's going on with your system... there are some problems you need to be right in front of and this would be one of them.

Thanks, Allan.

I was beginning to believe I had landed in the twilight zone.

The system BIOS is basically checking your hardware and, othen than the boot sequence settings, has nothing to do with a systems ability to boot.
http://en.wikipedia.org/wiki/BIOS

Then the OS begins to load
http://www.webopedia.com/DidYouKnow/Hardware_Software/2004/BootProcess.asp

After that you have your deskktop and a functional computer.
It is really quite a complicateds process to be performed in about under a minute

What I do not see you attempting is to boot from a genuine WIndows XP cd.
I dont know if any of the disks you are using really are bootable.
UBCD should be if made correctly

Try a genuine XP cd

rabbitHutch, on 02 December 2011 - 12:35 PM, said:

Does your Computer was Dell?

If your Computer was not Dell then you problem was between your Hardrive cable (power/data) or your Hardrive was failing.

And please dont waste your time to try run fixboot or any command on Recovery Console, it's better to run a Hardrive Brand diagnostic utility to check your Hardrive.

Eyesee, on 02 December 2011 - 10:20 PM, said:

the bios has everything to do with the computer booting ,without the correct bios info for said computer , the computer will never boot ,just try booting one with the wrong bios version or a corrupt bios,the work "DEAD" comes to my mind
to steal a quote i read many years ago, "The bios is to a computer' what a pair of skates are to a skater "

This post has been edited by caperjac: 03 December 2011 - 07:11 AM

Okay, I think we're getting into semantics and probably just a slight mis-statement by Eyesee. The bottom line is that the hard drive (and thus the MBR) is not accessed to enter bios or to boot to external media (ie, a cd). But yes, correct settings in the bios are indeed required to boot the system.