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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?

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  • remove your device-map file and rerun grub-install
    – Michael Martinez
    Apr 14, 2014 at 22:21
  • I'm loading up the rescue shell to try that now, but could you explain how that will work? As I understand it, the contents of my device.map are fine.
    – Elliot B.
    Apr 14, 2014 at 22:32
  • grub-install will re-read the devices, so it's the easiest first thing to try. If that doesn't work, then the problem is either (a) bad network connection during some part of the install; or (b) bad hardware.
    – Michael Martinez
    Apr 14, 2014 at 22:34
  • I get the same result as before when I run grub-install /dev/sda without removing device.map. I rebooted, grub prompt is gone, but in its place is an Error 21 message -- selected disk does not exist. I've experienced this error on two completely different sets of hardware (identical server configuration though). I suppose a network issue could be to blame, but I've repeated the CentOS install so many times that I doubt that's the issue. Ubuntu net-install works fine.
    – Elliot B.
    Apr 14, 2014 at 22:54
  • zero out the first couple megs on each hard drive and try again. if it still does'nt work, then run tests on the RAM, hard drives and motherboard of the two servers in question.
    – Michael Martinez
    Apr 14, 2014 at 23:11

1 Answer 1

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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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  • Unfortunately this didn't work for me, I'm now reformatting the entire drive instead, and will install centOS 6.9 yet again.
    – Jason
    Jul 19, 2019 at 10:49

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