One of the deficiencies in GRUB was the tendency for Linux experimenters to start with a Windows PC, install Linux to a completely different unused primary partition, and end up with GRUB installed to the drive the Linux partition was on, rather than the Linux partition itself.

This overwrote the generic (DOS/Windows) MBR on sector 0 with a Grub-specific MBR which hard-linked to more Grub-specific code stored on otherwise unused sectors located in between sector 0 and the first sector of the (not necessarily formatted) first partition, expected to be at least 31 sectors available for Grub to install to. And for Grub to have those sectors to itself (not true if other apps treated it as scratch space outside the file system).

You could link Grub to other sectors if you wanted to but this was where the default location was.

You could also always have a dedicated boot partition but this did not become mainstream until Windows 7 and it was with BIOS booting. That approach has now been continued into UEFI with the dedicated EFI folder on the hidden ESP partition.

So Grub took over your PC and that's how it was going to boot. Most of the time in Grub1 you had to carefully manually edit the grub.conf file to get your old untouched Windows partition on the boot menu so your dual-boot layout was ideally functional.

This didn't always happen during a user's first Linux experience so the HDD was basically stuck in "Linux only" until they figured it out. If the urgent need to boot the old Windows came up, reboot to the emergency boot floppy containing FDISK.EXE, then run FDISK /MBR and everything would be back to "Windows only" after that.

With Grub2 it kind of got worse since grub.cfg came along in place of grub.conf, and grub.cfg is intended to be autogenerated and not manually edited. The recommended recommendation is to never manually edit the grub.cfg file. I recommend not always doing what's recommended if you want to really get what you want. But don't fail to back up your work in case it autodisappears. In the long run you could really get into Grub2 and make it autogenerate your dreams.

Either way it was much better when installing Linux to a type 83 partition to leave the generic DOS/Windows MBR in position at sector 0, and instead direct Grub to be installed to the same partition (that partition's bootsector) that Linux was destined for. This made the (EXTx-formatted) type 83 Linux partition completely bootable on its own. But only if that was a primary partition to begin with and was flagged active with a binary 80 in its partition table entry in sector 0. And at the time the Windows partition would instead have a 00 flag, therefore not being active and was not the primary to boot that time.

You would have to manually assign which primary partition to flag active so you could multiboot, but it is accomplished that way without even a menu to configure. This was best done manually with a binary disk editor although some menu-interface shells arose such as Bootmagic which handled self-contained bootable partitions especially well. Boot magic also usually installed to "unused" sectors on the HDD outside any file system.

Further, the functional grub bootsector for the partition it is installed to can be copied to a binary file (named something like bootsect.lin) on an active FAT or NTFS partition where an NT install had often autosaved a copy of a FAT DOS bootsector as bootsect.dos. (Before it was replaced with an NT bootsector).

BOOT.INI on the active Windows bootup partition could then be edited to include a boot entry invoking bootsect.lin and the NT bootloader would jump to the target type 83 Linux partition and carry on from there.

So you could install and boot Linux ideally to an extra partition on a Windows PC without disturbing the Windows in any way. Just adding the Linux option to the default Windows boot menu.

When a small separate FAT32 boot partition began to appear before the main NTFS volume on OEM Win7 PC's, that's when Syslinux began to really shine. Because Syslinux is made to boot Linux from an active FAT partition similar to how Isolinux boots Linux from CDROM. Syslinux has its own kind of bootsector and relies on a syslinux folder and/or syslinux.cfg file so for a while there it was a better choice than Grub. You just had to use the same version of syslinux that the distro was isolinux'ed with, and rename the isolinux folder & isolinux.cfg file to syslinux & syslinux.cfg, with judicious manual editing.

Syslinux is stll superior for BIOS booting but for UEFI Grub2 is what seems to work most predictably now.

Under UEFI Grub2 acts about the same way always booting from the dedicated EFI folder on the hidden FAT32 ESP boot partition. It simply overwrites \EFI\boot\bootx64.efi with a distro-specific version from the distro that you are booted to when you update grub. For instance if you install Ubuntu to a Linux-formatted GPT volume and properly direct Grub to install in the EFI folder it makes a new bootfolder \EFI\ubuntu in the process and the new \EFI\boot\bootx64.efi no longer seeks \EFI\Microsoft, it seeks \EFI\ubuntu instead (where it finds Ubuntu's grub.cfg). But it leaves \EFI\Microsoft untouched, and each additional distro adds its own like \EFI\fedora and \EFI\debian, and they all coexist wonderfully.

Don't let different mainboard UEFI implementations fool you, the firmware itself is supposed to recognize different boot options as they arise and you can get a firmware boot entry (available from a separate usually disabled mainboard UEFI bootmenu) for each installed OS, which all may not be functional choices after a while. But \EFI\Microsoft\bootmgfw.efi can be like a hardcoded firmware boot entry as a failsafe alternative (Windows boot manager) to an expected \EFI\boot\bootx64.efi. Either of which \EFI\boot\bootx64.efi or \EFI\Microsoft\bootmgfw.efi are further alternatively executable from a UEFI shell. Whether the UEFI shell is built into the mainboard firmware, or if not you have to provide shellx64.efi on a pre-boot (usually FAT32) file device.

Whichever distro you are booted to will be the one whose grub.cfg is reautogenerated when you update grub. You are expecting the grub update process to autodetect all installed OS's including Windows and place entries for them on a boot menu in grub.cfg which appears routinely every time you restart the PC. Some distros do this better than others.

So it's a good idea to back up the Windows version of bootx64.efi for possible restoration to \EFI\boot before allowing Grub to overwrite it with a Linux version. Then again you might want to back up your Linux versions of bootx64.efi as they come along too.

Pretty much the modern substitute for an FDISK /MBR, although the BCDBOOT command (sometimes requiring the /uefi option) overwrites \EFI\boot\bootx64.efi with a fresh Windows version, while it examines your \EFI\Microsoft\boot\BCD for any corrections it wants to make.

Just to bring things more up-to-date, it's not 1999 any more.