By "multi-booting", I mean choosing from a range of Operating Systems (OSs) at boot time, without having to insert boot disks. It helps if you understand the layers at which choices can be made, and how partitions, volumes and drives relate to each other.
Why would you want to host multiple OSs on the same computer?
Not all of these needs may require installing a different OS to the hard drive. You may find older software works within the main OS once a few issues or settings are sorted out, that your maintenance OS is reliable enough when booted from something other than the hard drive, or you feel comfortable with testing dubious software in your main environment (good luck!).
I'll be honest with you; it's quite possible to get yourself tied up in knots, with OSs corrupting each other, drive letters getting the wrong letters assigned, and finding yourself unable to boot certain OSs or anything at all. So you'd want to:
This article assumes you are setting up these OSs from scratch. If you want to retro-fit an alternate OS such as a HD-based DOS mode for WinME or NT, or restore access to a previously-installed OS after installing a newer one, then the approach would differ.
Which layer approaches can you use?
The "cheapest" layer is to have both OSs residing in the same partition. That can only work if the OSs are similar enough to read each other's file systems, and yet different enough not to overlap via use of the same directories or file names.
A cleaner layer that can accommodate very different OSs is to use different partitions on the same physical hard drive.
The same can work for OSs that are the same or similar, though you are more likely to need an add-on boot manager such as Boot-It New Generation (BING) from www.bootitng.com to manage the OS selection and hide the OSs volumes from each other. You may also need to install the boot manager to break the 4 partition limit imposed by the standard Master Boot Record code.
An alternate approach is to emulate an older OS (or one that requires completely different hardware) within your main OS.
Check the details on limitations and caveats that apply to each of Microsoft's OSs. Generally:
NT + Win9x DOS mode or MS-DOS
can share a C:
- C: must be FATxx
- C: must be FAT16 for MS-DOS or pre-SR2.x Win95 DOS
mode
- prevent NT from interpreting Config.sys and Autoexec.bat
- best approach is to install the DOS (mode), then the NT
Dual installations of the same
version of Win9x can share a C: if:
- you swap in the correct Windows and "Program Files"
subtrees
- you suppress System Restore
if WinME
Dual installations of the different
version of Win9x can share a C: if:
- C: must be FAT16 for pre-SR2.x Win95
- you swap in the correct Windows and "Program Files"
subtrees
- you swap in the correct boot code files
- you suppress System Restore
if WinME
NT + Win9x GUI mode is best done on
different hard drives or volumes
- you may be able to use the above approach on one C:
- even so, it's safest to keep these separate
- the bulk of one or both OSs can be on a volume other than
C:
- but C: must be visible to both OSs
- best approach is to install the Win9x, then the NT (Boot.ini)
NT + NT mode is best done on
different hard drives or volumes
- the bulk of one or both OSs can be on a volume other than
C:
- but C: must be visible to both OSs
- best approach is to install the older NT first (Boot.ini)
- hide different NT versions from each other, esp. if NTFS
Should inactive OSs be hidden?
Sometimes one OS can corrupt another's installation or file system, e.g.:
For this reason, you may need to hide OS installations while they are not running, typically by spoofing the partition type byte to a value unknown to the OS that is running. You may need 3rd-party tools such as Boot-It NG to do this.
(C) Chris Quirke, all rights reserved - April 2004