A "how" and "why" question

[garys1n2]

This isn't a question as to how to fix something, but more of how it happens and why, so I guess there really wouldn't be a set answer, only theories and opinions, but here goes"

Recently, I've had the "NTDLR is missing" problem on my computer, and I went through the various fixes mentioned here and elsewhere about trying to load from a corrupt/missing/etc. hard drive, and they worked (which is why I am able to post here), but what I don't understand is how or why the boot drive in the Bios can change without any manual changes made by me. I am running multiple hard drives on this computer, and it seemed like overnite, the boot hard drive changed from the C drive to another. Through plugging in and unplugging, and restarting the computer, I was able to determine which HD was the C drive, and now all is fine (I also labled the hard drives as to which was which on the hard drive case itself), but I dont understand how the boot drive was replaced with a non-boot drive which gave me the NTDLR error message.

Ya'll's expertise would be greatly appreciated in helping me understand how the boot drive changed without human interference. Thanks in advance. (Like I said, its not a how do I fix question, but more of how does it happen question).

[kilgoretrout]

Your bios doesn't know from A, B or C drives. It only knows what drive has been set in the bios setup as the first one to try and boot. If that drive is determined to be not bootable by the bios, it goes to the second drive listed in the boot sequence and tries again and so on until it runs out of devices.

If you are saying that the boot sequence changed in your bios setup without any action by you, then the only thing I can think of is the battery on your motherboard may be going as that can cause you to lose your bios settings.

[EzyStvy]

Another example: If you dual boot an OS and XP AND you installed XP last - XP copies files like NTDLR to the first OS's partition/drive....If you remove that first partition/drive the files are no longer there and thus become "missing."

[mikeL]

You can also get the ntldr error if you edit the boot.ini file and it points to the wrong hardrive / partition location.