Multi boot or Multi booting (usually dual booting, but many OSes can be booted from the same computer) is the act of installing multiple operating systems on a computer, and being able to choose which one to boot when switching on the computer power. The program which makes multi booting possible is called a boot loader.grub_with_ubuntu_and_windows_vista1

Multi booting is useful in many situations, such as those where several pieces of software require different operating systems and cannot be run on a single system. A multi boot configuration will allow a user to use all of this software on one computer. Another reason for setting up a multi boot system can be that one wants to investigate or test a new operating system without switching completely. Multi booting allows one to get to know the new system, configure all applications needed and migrate data before making the final step and removing the old operating system. This is often accomplished by using a boot loader that can boot more than one operating system, such as NTLDR, LILO, or GRUB.

Multi booting can also aid software developers where multiple operating systems are required for development or testing purposes. Having these systems on one machine can greatly reduce hardware costs. (However hardware costs are counterbalanced by system management costs, and the costs of the unavailability of the software that cannot be run at any given moment. Another solution to these problems is to use virtual machine software to emulate another computer from within the operating system of choice.)

A popular multi-boot configuration is a mixed-OS system in which Linux is one of the secondary (or primary) installations. In terms of business strategy, Windows does not facilitate or support multi-boot systems, other than allowing for partition-specific installations, and no choice of boot loader is offered. To deal with such installs requires consultation with Linux afficionados and techs, who are typically well-versed in the concept.

The basic concept involves partitioning a disk, to accommodate each planned installation, including separate partitions for data storage or backups. The partitions should be done with a Windows partitioning tool (diskpart, Disk Management), rather than a Linux tool (parted, QTparted), for the simple reason that Windows is more particular (cf. “picky”) about how the partition table is written and will occasionally complain or even show errors if its installed to a Linux-created (or sometimes modified) partition table. Linux tools are powerful, (ie. shrinking an NTFS drive) but Windows has particularities which must be considered. (See master boot record and extended boot record).

Windows should be installed to the first primary drive. Though Windows can be installed to another drive, certain particularities (drive letter assignments, expected system partition number) can make such installations problematic, while Linux installations on primary or logical drives have no such problems whatsoever.

The boot manager/loader should be installed by the Linux distribution. All Windows installations will be easily found by Linux, but Windows boot managers do not find Linux installations (nor does Windows deal natively with Linux file systems).

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Excellent resources below:

How to dual boot Windows XP and Linux (XP installed first) — the step-by-step guide with screenshots

Illustrated Dual Boot Site – Awesome site with all things Linux!