Configuring Grub 2 on CentOS 7 to Dual Boot with Windows 7:

• Once you install CentOS 7 alongside your Windows OS, you may find that you cannot boot into Windows. The Grub bootloader may only show your Linux OS as your only options to boot from.

• To fix this and have the Grub bootloader list your Windows OS, you need to edit the Grub bootloader files.
• If you have used CentOS is the past (with 6 or earlier), you may find that editing Grub is different. Previously, you would edit /boot/grub/grub.conf.
• This is no longer the case, as the grub2.cfg file is generated dynamically, based on dependency files. Here’s what you need to edit to configure your bootloader.

STEPS:1

• Boot into CentOS 7.

STEPS:2

• Determine what partition your Windows OS resides on by running sudo fdisk -l in Terminal.

Disk /dev/sda: 320.1 GB, 320072933376 bytes, 625142448 sectors
 Units = sectors of 1 * 512 = 512 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disk label type: dos
 Disk identifier: 0xcd8b1219

 Device Boot          Start              End              Blocks 	  Id  	System
 /dev/sda1   *        2048          4194303       2096128   	 7  	HPFS/NTFS/exFAT
 /dev/sda2         4194304      360402758   178104227+           7              HPFS/NTFS/exFAT
 /dev/sda3       360402942   625141759   132369409              5               Extended
 /dev/sda5       612595712   625141759     6273024                82           Linux swap / Solaris
 /dev/sda6       360407040   361431039      512000                 83               Linux
 /dev/sda7       361433088   612589567   125578240              8e             Linux LVM

In this example, /dev/sda1 is the recovery partition, and /dev/sda2 is the Windows OS partition. Since partition indexes start at zero, the Windows OS partition will be `hd0,1` (a = 0, 2 = 1; or first disk, second partition) when we edit the Grub file. Make note of this.

STEPS:3

• Open a terminal and navigate to /etc/grub.d/:

cd  /etc/grub.d/

STEPS:4

• Edit the 40_custom file. You may not see the file if you ls in /grub.d/.

sudo nano 40_custom

STEPS:5

• You should see the following in the nano text editor:

#!/bin/sh
 exec tail -n +3 $0
 # This file provides an easy way to add custom menu entries.  Simply type the
 # menu entries you want to add after this comment.  Be careful not to change
 # the 'exec tail' line above.

STEPS:6

• Below the last #, type on a new line:

menuentry "Windows 7" {
         set root=(hd0,1)
         chainloader +1
         }

STEPS:7

• Finally, run the following to apply the changes to the grub.cfg file:

grub2-mkconfig --output=/boot/grub2/grub.cfg

• Once you reboot, you should see the option of booting into Windows 7.
• If a default boot entry into Windows (or something else) is requested, then you need to edit the GRUB_DEFAULT in /etc/default/grub:

GRUB_DEFAULT="Windows 7"