hii! I had dual booted my windows 8.1 with RHEL7.3 ,so after doing that when i restarted my system windows boot option was gone from the bootloader and till now i have tried the below steps-

1)I have tried command 'grub2-mkconfig'but windows is not listed

2)Then i tried to add a manual entry for windows in /etc/grub.d/40_custom
My 40_custom file looks like (followed from https://access.redhat.com/documentat...tion_File.html)
#!/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.
menuentry "Windows 8.1"{
set root='(hd0,4)'
chainloader +1
}
After making changes i have rebooted and selected windows8.1 option but it gave an error about efi.

-----I have install bootinfoscript and its RESULTS.txt file looks like-----
Boot Info Script 0.61 [1 April 2012]


============================= Boot Info Summary: ===============================

=> Windows is installed in the MBR of /dev/sda.

sda1: __________________________________________________________________________

File system: ntfs
Boot sector type: Windows Vista/7: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files:

sda2: __________________________________________________________________________

File system: vfat
Boot sector type: FAT16
Boot sector info: According to the info in the boot sector, sda2 starts
at sector 0. But according to the info from fdisk,
sda2 starts at sector 1333248.
Operating System:
Boot files:

sda3: __________________________________________________________________________

File system:
Boot sector type: -
Boot sector info:
Mounting failed: mount: unknown filesystem type ''

sda4: __________________________________________________________________________

File system: ntfs
Boot sector type: Windows Vista/7: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files: /bootmgr /Windows/System32/winload.exe

sda5: __________________________________________________________________________

File system: ntfs
Boot sector type: Windows Vista/7: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files:

sda6: __________________________________________________________________________

File system: xfs
Boot sector type: -
Boot sector info:
Operating System:
Boot files: /grub2/grub.cfg

sda7: __________________________________________________________________________

File system: ntfs
Boot sector type: Windows Vista/7: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files:

sda8: __________________________________________________________________________

File system: ntfs
Boot sector type: Windows Vista/7: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files: /bootmgr /boot/BCD

sda9: __________________________________________________________________________

File system: ntfs
Boot sector type: Windows Vista/7: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files:

sda10: _________________________________________________________________________

File system: xfs
Boot sector type: -
Boot sector info:
Operating System: Kernel on an
Boot files: /etc/fstab

sda11: _________________________________________________________________________

File system: swap
Boot sector type: -
Boot sector info:

============================ Drive/Partition Info: =============================

Drive: sda _____________________________________________________________________

Disk /dev/sda: 1000.2 GB, 1000204886016 bytes, 1953525168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Partition Boot Start Sector End Sector # of Sectors Id System

/dev/sda1 1 1,953,525,167 1,953,525,167 ee GPT


GUID Partition Table detected.

Partition Start Sector End Sector # of Sectors System
/dev/sda1 2,048 1,333,247 1,331,200 Windows Recovery Environment (Windows)
/dev/sda2 1,333,248 1,865,727 532,480 EFI System partition
/dev/sda3 1,865,728 2,127,871 262,144 Microsoft Reserved Partition (Windows)
/dev/sda4 2,127,872 508,819,455 506,691,584 Data partition (Windows/Linux)
/dev/sda5 508,819,456 757,547,007 248,727,552 Data partition (Windows/Linux)
/dev/sda6 757,547,008 758,079,487 532,480 Data partition (Windows/Linux)
/dev/sda7 962,349,056 1,910,571,007 948,221,952 Data partition (Windows/Linux)
/dev/sda8 1,910,573,056 1,953,513,471 42,940,416 Data partition (Windows/Linux)
/dev/sda9 759,644,160 761,741,311 2,097,152 Data partition (Windows/Linux)
/dev/sda10 761,741,312 925,581,311 163,840,000 Data partition (Windows/Linux)
/dev/sda11 925,581,312 927,678,463 2,097,152 Swap partition (Linux)

"blkid" output: ________________________________________________________________

Device UUID TYPE LABEL

/dev/sda1 98C2079EC2077FAC ntfs WINRE
/dev/sda10 e388f7c9-8e0a-403c-adb3-984883093a78 xfs
/dev/sda11 c9fa50b6-6db2-4e02-ae9b-37c98d6bc1e1 swap
/dev/sda2 D813-7BA4 vfat
/dev/sda3
/dev/sda4 50E4C5B7E4C5A018 ntfs
/dev/sda5 760861080860C927 ntfs New vol
/dev/sda6 6f394d65-391b-4f7c-93ee-bcff9426a3d3 xfs
/dev/sda7 AEE42754E4271DD9 ntfs New Volume
/dev/sda8 4E8405CF8405BB0B ntfs RECOVERY
/dev/sda9 24C23311C232E728 ntfs
/dev/sr0 2015-11-04-11-25-00-0 iso9660 iB-WUA300NM

================================ Mount points: =================================

Device Mount_Point Type Options

/dev/sda10 / xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/sda10 /var/lib/docker/overlay xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/sda2 /boot/efi vfat (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=ascii,shortname=winnt,errors=remount-ro)
/dev/sda6 /boot xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/sr0 /run/media/root/iB-WUA300NM iso9660 (ro,nosuid,nodev,relatime,uid=0,gid=0,iocharset=utf8,mode=0400,dmode=0500,uhelper=udisks2)


============================= sda6/grub2/grub.cfg: =============================

--------------------------------------------------------------------------------
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
set pager=1

if [ -s $prefix/grubenv ]; then
load_env
fi
if [ "${next_entry}" ] ; then
set default="${next_entry}"
set next_entry=
save_env next_entry
set boot_once=true
else
set default="${saved_entry}"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
menuentry_id_option="--id"
else
menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
set saved_entry="${prev_saved_entry}"
save_env saved_entry
set prev_saved_entry=
save_env prev_saved_entry
set boot_once=true
fi

function savedefault {
if [ -z "${boot_once}" ]; then
saved_entry="${chosen}"
save_env saved_entry
fi
}

function load_video {
if [ x$feature_all_video_module = xy ]; then
insmod all_video
else
insmod efi_gop
insmod efi_uga
insmod ieee1275_fb
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
fi
}

terminal_output console
if [ x$feature_timeout_style = xy ] ; then
set timeout_style=menu
set timeout=5
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
set timeout=5
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/00_tuned ###
set tuned_params=""
### END /etc/grub.d/00_tuned ###

### BEGIN /etc/grub.d/01_users ###
if [ -f ${prefix}/user.cfg ]; then
source ${prefix}/user.cfg
if [ -n "${GRUB2_PASSWORD}" ]; then
set superusers="root"
export superusers
password_pbkdf2 root ${GRUB2_PASSWORD}
fi
fi
### END /etc/grub.d/01_users ###

### BEGIN /etc/grub.d/10_linux ###
menuentry 'Red Hat Enterprise Linux Server (3.10.0-514.el7.x86_64) 7.3 (Maipo)' --class red --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-514.el7.x86_64-advanced-e388f7c9-8e0a-403c-adb3-984883093a78' {
load_video
set gfxpayload=keep
insmod gzio
insmod part_gpt
insmod xfs
set root='hd0,gpt6'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt6 --hint-efi=hd0,gpt6 --hint-baremetal=ahci0,gpt6 6f394d65-391b-4f7c-93ee-bcff9426a3d3
else
search --no-floppy --fs-uuid --set=root 6f394d65-391b-4f7c-93ee-bcff9426a3d3
fi
linuxefi /vmlinuz-3.10.0-514.el7.x86_64 root=UUID=e388f7c9-8e0a-403c-adb3-984883093a78 ro crashkernel=auto rhgb quiet
initrdefi /initramfs-3.10.0-514.el7.x86_64.img
}
menuentry 'Red Hat Enterprise Linux Server (0-rescue-dfe9befd7f824e5c9cbf0cc322862b7a) 7.3 (Maipo)' --class red --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-dfe9befd7f824e5c9cbf0cc322862b7a-advanced-e388f7c9-8e0a-403c-adb3-984883093a78' {
load_video
insmod gzio
insmod part_gpt
insmod xfs
set root='hd0,gpt6'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt6 --hint-efi=hd0,gpt6 --hint-baremetal=ahci0,gpt6 6f394d65-391b-4f7c-93ee-bcff9426a3d3
else
search --no-floppy --fs-uuid --set=root 6f394d65-391b-4f7c-93ee-bcff9426a3d3
fi
linuxefi /vmlinuz-0-rescue-dfe9befd7f824e5c9cbf0cc322862b7a root=UUID=e388f7c9-8e0a-403c-adb3-984883093a78 ro crashkernel=auto rhgb quiet
initrdefi /initramfs-0-rescue-dfe9befd7f824e5c9cbf0cc322862b7a.img
}
if [ "x$default" = 'Red Hat Enterprise Linux Server (3.10.0-514.el7.x86_64) 7.3 (Maipo)' ]; then default='Advanced options for Red Hat Enterprise Linux Server>Red Hat Enterprise Linux Server (3.10.0-514.el7.x86_64) 7.3 (Maipo)'; fi;
### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/20_ppc_terminfo ###
### END /etc/grub.d/20_ppc_terminfo ###

### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/40_custom ###
# 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.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f ${config_directory}/custom.cfg ]; then
source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f $prefix/custom.cfg ]; then
source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###
--------------------------------------------------------------------------------

=================== sda6: Location of files loaded by Grub: ====================

GiB - GB File Fragment(s)


=============================== sda10/etc/fstab: ===============================

--------------------------------------------------------------------------------

#
# /etc/fstab
# Created by anaconda on Sun Jun 18 06:17:00 2017
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
UUID=e388f7c9-8e0a-403c-adb3-984883093a78 / xfs defaults 0 0
UUID=6f394d65-391b-4f7c-93ee-bcff9426a3d3 /boot xfs defaults 0 0
UUID=D813-7BA4 /boot/efi vfat umask=0077,shortname=winnt 0 0
UUID=c9fa50b6-6db2-4e02-ae9b-37c98d6bc1e1 swap swap defaults 0 0


--------------------------------------------------------------------------------

=============================== StdErr Messages: ===============================

cat: /tmp/BootInfo-ygyhMi6M/Tmp_Log: No such file or directory
mdadm: No arrays found in config file or automatically

Please someone tell me what to do because i need my windows back ASAP.I have tried nearly all the answers given on internet but nothing works.

Simplest is to merely get into the boot options (maybe F10 or F12, watch the boot splash screen) and pick Windows from there - like getting into a BIOS list at power-on.
That entry you created is for an BIOS/MBR system, not UEFI/gpt. Here are the relevant bits from a UEFI system that was generated by os-prober.You need to reference the EFI partition. Recreate the .cfg and see how you go.

1 members found this post helpful.

Hey i just tried to write this code snippet in my 40_custom file...It gave me a windows option but on selecting it, gave me error message. What should i do now should i put this code in os_prober and if yes where to put it in that file.