I have confirmed that the USB has a proper bootable Ubuntu ISO on it with the file command.

USB is plugged in the laptop.

The grub menu does get shown on its own at the startup.

Problem is that there is no option to boot from USB.

Now I want to edit grub.cfg such that it contains USB boot options.

What do I do from here?

Also, /boot/grub/grub.cfg:

#

# DO NOT EDIT THIS FILE

#

# It is automatically generated by grub-mkconfig using templates

# from /etc/grub.d and settings from /etc/default/grub

#



### BEGIN /etc/grub.d/00_header ###

if [ -s $prefix/grubenv ]; then

  set have_grubenv=true

  load_env

fi

if [ "${initrdfail}" = 2 ]; then

   set initrdfail=

elif [ "${initrdfail}" = 1 ]; then

   set next_entry="${prev_entry}"

   set prev_entry=

   save_env prev_entry

   if [ "${next_entry}" ]; then

      set initrdfail=2

   fi

fi

if [ "${next_entry}" ] ; then

   set default="${next_entry}"

   set next_entry=

   save_env next_entry

   set boot_once=true

else

   set default="0"

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 initrdfail {

    if [ -n "${have_grubenv}" ]; then if [ -n "${partuuid}" ]; then

      if [ -z "${initrdfail}" ]; then

        set initrdfail=1

        if [ -n "${boot_once}" ]; then

          set prev_entry="${default}"

          save_env prev_entry

        fi

      fi

      save_env initrdfail

    fi; fi

}

function recordfail {

  set recordfail=1

  if [ -n "${have_grubenv}" ]; then if [ -z "${boot_once}" ]; then save_env recordfail; fi; 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

}



if [ x$feature_default_font_path = xy ] ; then

   font=unicode

else

insmod part_gpt

insmod ext2

set root='hd0,gpt2'

if [ x$feature_platform_search_hint = xy ]; then

  search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

else

  search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

fi

    font="/usr/share/grub/unicode.pf2"

fi



if loadfont $font ; then

  set gfxmode=auto

  load_video

  insmod gfxterm

  set locale_dir=$prefix/locale

  set lang=en_IN

  insmod gettext

fi

terminal_output gfxterm

if [ "${recordfail}" = 1 ] ; then

  set timeout=30

else

  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

fi

### END /etc/grub.d/00_header ###



### BEGIN /etc/grub.d/05_debian_theme ###

set menu_color_normal=white/black

set menu_color_highlight=black/light-gray

### END /etc/grub.d/05_debian_theme ###



### BEGIN /etc/grub.d/10_linux ###

function gfxmode {

    set gfxpayload="${1}"

    if [ "${1}" = "keep" ]; then

        set vt_handoff=vt.handoff=7

    else

        set vt_handoff=

    fi

}

if [ "${recordfail}" != 1 ]; then

  if [ -e ${prefix}/gfxblacklist.txt ]; then

    if hwmatch ${prefix}/gfxblacklist.txt 3; then

      if [ ${match} = 0 ]; then

        set linux_gfx_mode=keep

      else

        set linux_gfx_mode=text

      fi

    else

      set linux_gfx_mode=text

    fi

  else

    set linux_gfx_mode=keep

  fi

else

  set linux_gfx_mode=text

fi

export linux_gfx_mode

menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-e860e8f0-912f-4874-8f0f-97e343371f12' {

    recordfail

    load_video

    gfxmode $linux_gfx_mode

    insmod gzio

    if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi

    insmod part_gpt

    insmod ext2

    set root='hd0,gpt2'

    if [ x$feature_platform_search_hint = xy ]; then

      search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

    else

      search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

    fi

    linux   /boot/vmlinuz-5.13.0-27-generic root=UUID=e860e8f0-912f-4874-8f0f-97e343371f12 ro  quiet splash $vt_handoff

    initrd  /boot/initrd.img-5.13.0-27-generic

}

submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-e860e8f0-912f-4874-8f0f-97e343371f12' {

    menuentry 'Ubuntu, with Linux 5.13.0-27-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.13.0-27-generic-advanced-e860e8f0-912f-4874-8f0f-97e343371f12' {

        recordfail

        load_video

        gfxmode $linux_gfx_mode

        insmod gzio

        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi

        insmod part_gpt

        insmod ext2

        set root='hd0,gpt2'

        if [ x$feature_platform_search_hint = xy ]; then

          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

        else

          search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

        fi

        echo    'Loading Linux 5.13.0-27-generic ...'

        linux   /boot/vmlinuz-5.13.0-27-generic root=UUID=e860e8f0-912f-4874-8f0f-97e343371f12 ro  quiet splash $vt_handoff

        echo    'Loading initial ramdisk ...'

        initrd  /boot/initrd.img-5.13.0-27-generic

    }

    menuentry 'Ubuntu, with Linux 5.13.0-27-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.13.0-27-generic-recovery-e860e8f0-912f-4874-8f0f-97e343371f12' {

        recordfail

        load_video

        insmod gzio

        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi

        insmod part_gpt

        insmod ext2

        set root='hd0,gpt2'

        if [ x$feature_platform_search_hint = xy ]; then

          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

        else

          search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

        fi

        echo    'Loading Linux 5.13.0-27-generic ...'

        linux   /boot/vmlinuz-5.13.0-27-generic root=UUID=e860e8f0-912f-4874-8f0f-97e343371f12 ro recovery nomodeset dis_ucode_ldr 

        echo    'Loading initial ramdisk ...'

        initrd  /boot/initrd.img-5.13.0-27-generic

    }

    menuentry 'Ubuntu, with Linux 5.11.0-37-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.11.0-37-generic-advanced-e860e8f0-912f-4874-8f0f-97e343371f12' {

        recordfail

        load_video

        gfxmode $linux_gfx_mode

        insmod gzio

        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi

        insmod part_gpt

        insmod ext2

        set root='hd0,gpt2'

        if [ x$feature_platform_search_hint = xy ]; then

          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

        else

          search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

        fi

        echo    'Loading Linux 5.11.0-37-generic ...'

        linux   /boot/vmlinuz-5.11.0-37-generic root=UUID=e860e8f0-912f-4874-8f0f-97e343371f12 ro  quiet splash $vt_handoff

        echo    'Loading initial ramdisk ...'

        initrd  /boot/initrd.img-5.11.0-37-generic

    }

    menuentry 'Ubuntu, with Linux 5.11.0-37-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.11.0-37-generic-recovery-e860e8f0-912f-4874-8f0f-97e343371f12' {

        recordfail

        load_video

        insmod gzio

        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi

        insmod part_gpt

        insmod ext2

        set root='hd0,gpt2'

        if [ x$feature_platform_search_hint = xy ]; then

          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

        else

          search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

        fi

        echo    'Loading Linux 5.11.0-37-generic ...'

        linux   /boot/vmlinuz-5.11.0-37-generic root=UUID=e860e8f0-912f-4874-8f0f-97e343371f12 ro recovery nomodeset dis_ucode_ldr 

        echo    'Loading initial ramdisk ...'

        initrd  /boot/initrd.img-5.11.0-37-generic

    }

    menuentry 'Ubuntu, with Linux 5.8.0-43-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.8.0-43-generic-advanced-e860e8f0-912f-4874-8f0f-97e343371f12' {

        recordfail

        load_video

        gfxmode $linux_gfx_mode

        insmod gzio

        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi

        insmod part_gpt

        insmod ext2

        set root='hd0,gpt2'

        if [ x$feature_platform_search_hint = xy ]; then

          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

        else

          search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

        fi

        echo    'Loading Linux 5.8.0-43-generic ...'

        linux   /boot/vmlinuz-5.8.0-43-generic root=UUID=e860e8f0-912f-4874-8f0f-97e343371f12 ro  quiet splash $vt_handoff

        echo    'Loading initial ramdisk ...'

        initrd  /boot/initrd.img-5.8.0-43-generic

    }

    menuentry 'Ubuntu, with Linux 5.8.0-43-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.8.0-43-generic-recovery-e860e8f0-912f-4874-8f0f-97e343371f12' {

        recordfail

        load_video

        insmod gzio

        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi

        insmod part_gpt

        insmod ext2

        set root='hd0,gpt2'

        if [ x$feature_platform_search_hint = xy ]; then

          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2  e860e8f0-912f-4874-8f0f-97e343371f12

        else

          search --no-floppy --fs-uuid --set=root e860e8f0-912f-4874-8f0f-97e343371f12

        fi

        echo    'Loading Linux 5.8.0-43-generic ...'

        linux   /boot/vmlinuz-5.8.0-43-generic root=UUID=e860e8f0-912f-4874-8f0f-97e343371f12 ro recovery nomodeset dis_ucode_ldr 

        echo    'Loading initial ramdisk ...'

        initrd  /boot/initrd.img-5.8.0-43-generic

    }

}



### END /etc/grub.d/10_linux ###



### BEGIN /etc/grub.d/10_linux_zfs ###

### END /etc/grub.d/10_linux_zfs ###



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



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



### BEGIN /etc/grub.d/20_memtest86+ ###

### END /etc/grub.d/20_memtest86+ ###



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

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



### BEGIN /etc/grub.d/30_uefi-firmware ###

menuentry 'UEFI Firmware Settings' $menuentry_id_option 'uefi-firmware' {

    fwsetup

}

### END /etc/grub.d/30_uefi-firmware ###



### 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 ###