Du kannst jederzeit grub in den MBR neu installieren. Das geht beispielsweise mit einer Knoppix DVD/CD. Du mußt es nur so machen wie es im Handbuch zu grub (in english) steht:
http://www.gnu.org/software/grub/manual/
http://www.gnu.org/software/grub/manual/grub.html#Installing-GRUB-natively
3.2 Installing GRUB natively
Caution: Installing GRUB's stage1 in this manner will erase the normal boot-sector used by an OS.
GRUB can currently boot GNU Mach, Linux, FreeBSD, NetBSD, and OpenBSD directly, so using it on a boot sector (the first sector of a partition) should be okay. But generally, it would be a good idea to back up the first sector of the partition on which you are installing GRUB's stage1. This isn't as important if you are installing GRUB on the first sector of a hard disk, since it's easy to reinitialize it (e.g. by running `FDISK /MBR' from DOS).
If you decide to install GRUB in the native environment, which is definitely desirable, you'll need to create a GRUB boot disk, and reboot your computer with it. Otherwise, see Installing GRUB using grub-install.
Once started, GRUB will show the command-line interface (see Command-line interface). First, set the GRUB's root device4 to the partition containing the boot directory, like this:
grub> root (hd0,0)
If you are not sure which partition actually holds this directory, use the command find (see find), like this:
grub> find /boot/grub/stage1
This will search for the file name /boot/grub/stage1 and show the devices which contain the file.
Once you've set the root device correctly, run the command setup (see setup):
grub> setup (hd0)
This command will install the GRUB boot loader on the Master Boot Record (MBR) of the first drive. If you want to put GRUB into the boot sector of a partition instead of putting it in the MBR, specify the partition into which you want to install GRUB:
grub> setup (hd0,0)
If you install GRUB into a partition or a drive other than the first one, you must chain-load GRUB from another boot loader. Refer to the manual for the boot loader to know how to chain-load GRUB.
After using the setup command, you will boot into GRUB without the GRUB floppy. See the chapter Booting to find out how to boot your operating systems from GRUB.
Sollte Windows den MBR überschreiben mußt du nur diese drei Befehle in die grub shell eintippen:
1 grub starten:
grub
2. grub> find /boot/grub/stage1
3 Ausgabe diese letzen Befehls hier angeben:
grub> root (hd0,0)
4. grub in den MBR installieren:
grub> setup (hd0)
Man kann grub aber auch in die / Partition von Linux installieren und grub von einem anderem Bootmanager starten lassen:
grub> setup (hd0,0)
(hd0,0) entsprechend anpassen.
grub kann Windows nur indirekt mit Hilfe des chainloaders starten:
cat /boot/grub/menu.lst
.....
# This entry automatically added by the Debian installer for a non-linux OS
# on /dev/sda1
title Microsoft Windows 2000 Professional
root (hd0,0)
savedefault
makeactive
chainloader +1
# This entry automatically added by the Debian installer for a non-linux OS
# on /dev/sda2
title Microsoft Windows XP Professional
root (hd0,1)
savedefault
makeactive
chainloader +1
cscherwinski
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