Sstrhsrtj

I've gone through the usual steps for adding Windows to the bootloader options by modifying 40_custom and adding the following:

menuentry "Windows 10" {

     insmod part_gpt

     insmod chain

     set root='(hd0,msdos2)'

     chainloader +1

     }

However, when I select Windows in the bootloader, it says "Error: invalid signature. Press any key to continue". I don't know why I'm getting this error. How can I fix it?
EDIT: It says there are syntax errors here but I can't see any.
if [ "${grub_platform}" == "pc" ]; then
menuentry "Microsoft Windows Vista/7/8/8.1/10 BIOS/MBR" {
insmod part_msdos
insmod ntfs
insmod search_fs_uuid
insmod ntldr

search --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 XXXXXXXXXXXXXXXX
ntldr /bootmgr
}
fi

EDIT2: Output of sudo fdisk -l

~$ sudo fdisk -l

Disk /dev/sda: 298.1 GiB, 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

Disklabel type: dos

Disk identifier: 0x71b1e4fb



Device     Boot     Start       End   Sectors   Size Id Type

/dev/sda1              63     80324     80262  39.2M de Dell Utility

/dev/sda2  *    223580160 286285823  62705664  29.9G 83 Linux

/dev/sda3        30801920 223580159 192778240  91.9G  7 HPFS/NTFS/exFAT

/dev/sda4       286287870 625141759 338853890 161.6G  5 Extended

/dev/sda5       571742208 573741055   1998848   976M 82 Linux swap / Solaris

/dev/sda6       573743104 625141759  51398656  24.5G 83 Linux

/dev/sda7       286287872 345180159  58892288  28.1G 83 Linux



Partition table entries are not in disk order.





Disk /dev/sdb: 7.5 GiB, 8004829184 bytes, 15634432 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

Disklabel type: dos

Disk identifier: 0x037cbc77



Device     Boot Start      End  Sectors  Size Id Type

/dev/sdb1  *     2048 15634431 15632384  7.5G  c W95 FAT32 (LBA)

You should install os-prober and run it once

sudo apt-get install os-prober

sudo os-prober

Generate grub config file

sudo grub-mkconfig -o /boot/grub/grub.cfg

Don't edit the file manually

It looks like you've followed an old guide that expects Windows to use MBR partitioning.

The "invalid signature" error suggests Secure Boot is enabled. When Secure Boot is enabled, all bootloaders must be signed with a private key, and a matching public key must be included in the Secure Boot variables in the firmware NVRAM. GRUB is dutifully reading the first sector of the partition you've specified, but since it does not contain the appropriate Secure Boot signature, the firmware refuses to execute it.

Secure Boot requires native UEFI-style boot as a prerequisite. You may be able to disable Secure Boot on your system, but since GRUB already starts for you, there is probably no need to do that.

Your insmod part_gpt suggest you expect the disk to have a GPT-style partitioning which usually goes together with UEFI boot style, but on the other hand, set root=(hd0,msdos2) expects a MBR partition.

On my Debian system with GPT partitioning, the set root line reads: set root='hd0,gpt1'. If your system uses GPT partitioning, use the gptN partition identifiers instead of msdosN.

Also chainloader +1 tells GRUB to read the boot block from the first block of the partition; in UEFI native boot, there is no such thing. To boot Windows in UEFI mode, the set root line should point to the EFI System Partition that contains the Windows bootloader, and the chainloader line should be chainloader /EFI/Microsoft/Boot/bootmgfw.efi.