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Elliot B.
Asked: 2014-04-15 13:46:44 +0800 CST

Stuck at grub prompt after a default CentOS 6 installation

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I am having a very strange problem installing CentOS 6 that I've never encountered before. I've followed these same steps thousands of times before, but within the past couple weeks I've had two separate servers that would boot to a grub prompt after a fresh CentOS 6 install. Here are the steps of what I've done:

  1. I download the CentOS 6 net-install ISO to a Windows file-share.

  2. I use the IPMI web interface on my Supermicro server to mount the ISO as a USB virtual CD-ROM.

  3. I then proceed to install CentOS 6 with the default partitioning and default boot loader (installed into the MBR). At this point, everything is normal.

  4. I unmount the virtual CD-ROM and reboot the server to complete the installation. Instead of booting up CentOS, it goes to a GRUB version 0.97 prompt.

  5. I use the grub command root (hd0,0) to attempt to select the correct disk and then boot into the operating system, but when I type that command I get an Error 21 that the disk does not exist. I also try running root (hd1,0) thinking that the system labelled the primary drive as /dev/sdb during the installation. No such luck -- I get a disk read error message.

  6. At this point I reboot the server and use the install CD to get into rescue mode. I use rescue mode to inspect /boot/grub/device.map and /boot/grub/grub.conf -- everything is exactly as it should be -- (hd0) is mapped to /dev/sda and /boot/grub/grub.conf points to (hd0,0) for the boot partition. An fdisk -l command confirms that the boot partition is indeed setup on /dev/sda1

  7. For good measure, I ran grub-install /dev/sda to re-install grub and rebooted the server. After I did that, I was no longer able to get to the grub prompt -- instead my server would just display an Error 21 error message indicating the selected disk does not exist.

  8. I then proceeded to swap out hard drives and use a CentOS 6 net-install ISO hosted on a different mirror, repeated the steps above and got the same issue each time.

Screen capture of my grub.conf, device.map and fdisk -l from rescue shell: http://pbrd.co/1ijpSke

Screen capture failing to select (hd0,0) from grub prompt: http://pbrd.co/1ijr33d

I am able to install other operating systems on this server (I installed Ubuntu shortly thereafter without issue), but not CentOS 6.

I'm stumped where to go next. Any ideas?

linux

1 Answers

  1. Best Answer

    I eventually resolved this problem by following the advice given by Michael above. Zeroing out the first few megabytes of the drive and then re-installing the operating system did the trick. I suppose there was some sort of MBR or partition table corruption at play.

    If you are stuck at a grub screen after a fresh CentOS installation, try following these steps:

    1. Insert the CentOS installation disc or mount the ISO.
    2. Boot into rescue mode and enter into shell. Skip any steps to mount the existing file-system.
    3. Run the command fdisk -l to determine the label of the drive you need to zero out (e.g., /dev/sda, /dev/sdb). If you have multiple drives, be very careful to pick the right one.
    4. Run the command dd if=/dev/zero of=/dev/sdX bs=512 count=4000 where /dev/sdX is the drive in question (e.g., /dev/sda, /dev/sdb).
    5. Exit out of the rescue shell and reboot.
    6. Re-install CentOS 6 as per normal.
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