If there is a dual boot system ‘Windows-Linux’ or ‘Windows-Windows’, I mean in one HDD there are ‘a Windows 8.1 partition and a Windows 7 partition’ or ‘a Windows 8.1 partition and a Ubuntu partition’, now the question is if I want to delete one OS from the pair what I have to do? Suppose there is Windows 8.1-Windows 7 dual boot, I want to delete Windows 7 partition but I don’t want to lose Windows 8.1 partition, what can I do? If format the Windows 7 partition from Windows 8.1 then OS will be deleted but in time of booting option to choose one OS from the list will be shown. How to update this list? And if the boot record file is situated in Windows 7 partition then Windows 8.1 will also not be able to boot. Now I have to update the boot record in Windows 8.1 partition. But how can I do it? If the same situation happens with Windows-Linux dual boot system, then what to do?
closed as too broad by sawdust, fixer1234, DavidPostill♦, mdpc, Mike Fitzpatrick Jun 26 '15 at 5:52
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Windows boot files are either
on active partition in case of MBR disk
on EFI System Partition in case of GPT disk.
A. In the case of UEFI boot and GPT disk you can delete the partition of the OS you want to remove without problem.
A.1. If the OS to be removed is Windows - you have to delete also the corresponding BCD entry for loading that OS.
A.2. If the OS to be removed is Linux - you have to delete also the corresponding boot folder under \EFI on EFI System partition (e.g \EFI\ubuntu or \EFI\fedora)
B. In case of BIOS boot and MBR disk you cannot delete the active partition.
Here we have also 2 scenarios:
B.1. The OS to be deleted is on active. You must create boot files on another partition and make it active.
Now you can delete the old active partition and the OS on it.
B.2. The OS to be deleted is NOT on active.
Delete partition with the OS and amend BCD (boot configuration data) - delete corresponding loader object.
Note 1: The whole procedure for case B. assumes Windows boot manager is in control of the booting. If GRUB is in control then you first Repair Windows booting using "StartUp Repair" from recovery console then you delete Linux partitions (root and swap).
Note 2: You make all manipulations mentioned when booted to the Windows OS you want to keep except Windows boot repair mentioned in Note 1.