Tech Off Thread

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Why did this work?

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  • User profile image
    Shining Arcanine

    I had a Windows XP virtual machine in VirtualBox on a 3.5GB fixed-size virtual disk. I wanted to modify it to use a 20GB dynamically-sized virtual disk so that it would have room to grow without reserving all of the free space on my system's hard drive in the process, so I made a new virtual disk file, loaded a Linux LiveCD, copied the MBR and partition to the new image file, resized the partition and rebooted. Upon rebooting, I was greeted by the following boot loader error:

     

    A disk read error occurred
    Press Ctrl+Alt+Del to restart

     

    I did some google searches and discovered a site that suggested modifying byte 0x7E1A in a hex editor to be FF. Since my partition started at sector 128, it followed that I needed to modify 0x1001A. I made a few attempts at it, but the write buffer initially kept the change from being saved to the disk. Then I went through some of Microsoft's documentation and noticed two odd things.

     

    1. 0x1001A was 0x80, which is the value that the site said it would be.
    2. Microsoft's documentation said that the sector's value was meaningless, but its example had it being 0xFF, which is the value to which the site said it should be changed.
    I gave modifying 0x1001A another try, being careful to make sure that the change was saved to the disk, rebooted and the Windows bootloader works again.
    With that in mind, why does my Windows virtual machine work when Microsoft's documentation says that the change I made should not make a difference?

     

  • User profile image
    Dexter

    You seem to confuse the bootloader (which displays that error) with the NTFS filesystem driver which is what Microsoft's documentation is about. Ring any bells?

  • User profile image
    Shining Arcanine

    Dexter said:

    You seem to confuse the bootloader (which displays that error) with the NTFS filesystem driver which is what Microsoft's documentation is about. Ring any bells?

    I was googling BPB to try to find out what that byte did and that page is what I could find. I did another search now and found the following:

     

    http://en.wikipedia.org/wiki/BIOS_parameter_block

     

    It would have been nice if Microsoft could have said that it was not for NTFS, but did something elsewhere in Windows. :/

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