Andy's Unix FAQ

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Question:

Subject: boot loader
hi,

i installed redhat 7.3 in my computer with grub as my boot loader. i also installed windows xp. when i reboot my system my grub disappeared! how do i restore this?

thanks
gie

Answer:

This is a very common problem and is probably covered in a linux FAQ somewhere...

I guess you installed grub in the master boot record. Installation of recent windows variants (NT, 2000, 98, XP, ME) check the MBR and rewrite it if the contents seem corrupt. This is most likely how your grub installation got scrubed.

You need to boot a linux system from either a rescue floppy or your Redhat media ("rescue" boot). Then you can re-install grub. You should read the grub documentation on this ("info grub"), but for most systems this is simply a case of running;
# grub-install /dev/hda

If you were using Lilo as your boot loader the command would be;
# /sbin/lilo

TIP:
If you expect to have multiple operating systems on a PC (esp. if one is a MS one) it's better to install the boot loader (grub/lilo) in your linux partition, rather than the MBR. In this way Windows won't mess with it.

Restoring DOS/Windows MBR
If you arrived at this page looking for how to replace the grub/lilo MBR with the standard DOS version then you probably don't know about the DOS command;
     FDISK /MBR
This undocumented option to Fdisk causes it to re-write the MBR. Using this will make your Linux system unbootable but not irrecoverable - use a rescue floppy or CD.