Updating the distribution which is not running.

Many people like me have more than one Linux distributions on different hard disk partitions but only their current favourite is in use. How to update the other distribution which is not in use?

I am using Ubuntu Intrepid Ibex and would like to update/upgrade Arch Linux which is on the other partition. I am double clicking the partition and it gets mounted at /media/disk then I am using the following commands in the terminal:

$ sudo chroot /media/disk
bash: /dev/null: Permission denied
# pacman -Syu

I just ignore the error message and type the pacman -Syu command.

Arch Linux gets updated and upgraded without running it.