Boot from external harddrive

[maarth]

but must you install grub on the usbdrive or on the drive of the computer.

[900i]

Yes you can, just be sure to put the bootloader on the internal drive or the computer wont boot from the internal drive without the external one connected.

[yamawho]

Another way is to put GRUB on the usb drive and set it as the first hdd in the bios.
So if you remove it and boot, the system will just go back to the way it was booting from the internal.
If you some reason you need to boot to the internal and leave the usb connected, just use the boot menu by pressing F10 on most mobos, and select the hdd you want to boot from. If you use the boot menu, you don't really need to set the usb as the 1st hdd in the bios. I use this method to boot into Mandriva on my EeePC.

As usual, many ways to do something

[yamawho]

Many installers give you the option to install grub where you want.
You sometimes need to go into advanced mode during the install to have access.