Grub

GRUB is the Linux boot manager. When a new system is installed, GRUB needs to be installed to the harddisk and it has to be updated with the system type, kernel and partition in order to boot it correctly. If you add another OS to another partition, GRUB needs updating again so that it can list and boot that new OS.

Some options for installing/updating GRUB

1 ....Boot a trouble system with Super Grub2

http://www.supergrubdisk.org/

This might work if you hava an installed system that can't boot. Put Super Grub2 on a USB stick and boot it and it will scan the box for any installed system - just select one to boot it.

With the system up you can install GRUB and update it easily. As root,
where "sda" is the root of the hard disk you want to install GRUB on

My no.2 was a gui program called Grub customizer... but as I never use it any more, and it was dodgy with Debian (better suited for Ubuntu), I will leave it out as unnecessary bloatware...

2 ....Boot Repair Disk

http://sourceforge.net/projects/boot-repair-cd/

Run this Live CD and the Recovery app automatically searches for OS's present. Open the Advanced options to choose a location to install GRUB (set it to the root of your main harddisk, e.g. /dev/sda) and any other particulars.

This is a friendly GUI app for those unsure of a terminal.

You can install Boot repair to your system as well, but to detect and repair GRUB it must be run during a live session, and the version installed is arch specific, so 32bit will not repair GRUB on a 64bit system - as essentially it's chrooting.

1. get packages boot-repair and boot-sav - download from https://launchpad.net/ or https://launchpad.net/... or search for more recent ones.
2. install glad2script - download and install the .deb package from https://launchpad.net/glade2script

3. get and install parted https://packages.debian.org/...
4. install the boot-sav package with dpkg
5. finally install boot-repair package with dpkg

3 ...Chroot from a Live environment

You can chroot (change root) using a Live CD into the system that needs Grub installed...
and unmount the system with see my chroot page for more detail.

5 ....Using Rescue Mode of a net install CD

http://www.debian.org/...

type "rescue" at boot prompt ? or select Advanced Graphical Rescue Mode
when it arrives at rescue mode, select the mount point for the OS partition e.g. sda5
test the internet connection with

edit /etc/apt/sources.list if necessary, then install Grub

reboot

6 ....Using a custom GRUB entry

This is a last resort if other methods don't recognise the intalled systems.
https://wiki.archlinux.org/

then run: `sudo update-grub`
- OS prober will look for the OS in the custom entries file and add it to GRUB

Read up here
http://wiki.debian.org/GrubRecover
http://www.av8n.com/
easy-enough-recovery-from-debian-testing-installers-failure-to-install-grub-during-setup/

Removing a non-present OS from the GRUB menu

from a booted OS run: `sudo update-grub`

There are ways to set how the different systems on a box appear on the Grub menu.. see...

http://www.dedoimedo.com...
https://help.ubuntu.com/...