I have Arch Linux and Backtrack on my laptop. Both of them I think now are the harmonious couple.. I installed more 1 OS (dual boot), because dual boot is more secure than single boot. When 1 OS suddenly get error, we can use other OS for work. There is no guarantee when we use just 1 OS that there is no error, so when work we couldn't calm used just 1 OS.
After we have dual boot (or maybe multi boot), we can manage it from boot loader. The boot loader is the first software program that runs when a computer starts. It is responsible for loading and transferring control before initiating the boot process to the Operation System which we choose. The boot loader consists of GRUB, GRUB2, LILO, or Syslinux.
