I had dual boot Windows 8 and Fedora 28. The default OS is Fedora 28. In the dual boot screen I'm unable to see Windows 8. What should i do?

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There are two general tasks to complete:

1. Determine which disk and partition has the Windows 8 system volume expressed in Linux device naming conventions.
2. Edit the grub configuration to add an boot menu entry which boots that device and title it "Windows 8".

There are several ways to do each step.

If you prefer GUI, press the Windows Key† and type in "disks". This gives a display like:

The image is from my laptop which has only one physical drive containing three partitions, the simplest Fedora configuration.

Using the disks utility, you can poke around and find the Windows system boot partition. You can mount a potential candidate by pressing the "play button" on a partition then examine the files on that partition to tell if it has a WINDOWS and SYSTEM32 directories (and not your collections of music, photos, and videos).

If you prefer the command line, type sudo fdisk -l (lowercase L). The same system shows

Disk /dev/sda: 894.3 GiB, 960197124096 bytes, 1875385008 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: gpt
Disk identifier: C0A828D0-B398-498A-B026-8D91B5F29E09

Device       Start        End    Sectors   Size Type
/dev/sda1     2048     411647     409600   200M EFI System
/dev/sda2   411648    2508799    2097152     1G Linux filesystem
/dev/sda3  2508800 1875384319 1872875520 893.1G Linux LVM

Disk /dev/mapper/fedora-root: 50 GiB, 53687091200 bytes, 104857600 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 /dev/mapper/fedora-swap: 7.8 GiB, 8392802304 bytes, 16392192 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 /dev/mapper/fedora-home: 835.2 GiB, 896830275584 bytes, 1751621632 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

It has been a long time since I have had to do anything with grub's configuration. Hopefully someone else can answer that.

† The windows key is the center of these buttons: