grub> _ appears instead of starting ubuntu after deletion of snapshots in VirtualBox

[lesscaffee]

Problem:
Ubuntu operating system is not started. Instead of it the gnu grub bash-like terminal appears:

GNU GRUB version 2.02~beta2-9ubuntu1.12

Minimal BASH-like line editing is supported. For the first word, TAB lists possible command completions. Anywhere else TAB lists possible device or file completions.

grub> _

problem.png

This is shown if I do grub> ls (hd0<TAB>:
no-known-filesystem-detected.png

Before the grub terminal appears I was able to catch some errors:
errors-before-grub-terminal-is-shown.png

The VirtualBox menu for selection of boot devices can be triggered:
f12-for-boot-devices-menu.png


Steps applied which lead to problem (Before having applied these steps everything worked since months without problems!):

• Open Oracle VirtualBox Gui (v5.1.30).
• Select virtual machine.
• Go to tab "Snapshots".
• Delete step by step each existing snapshot.
• The deletion executed without failures (at least nothing poped up on the gui).
• Start virtual machine in the same Oracle VirtualBox GUI.
• Windows open and the grub terminal appears instead of loading the ubuntu operating system.

I already tried to create a new virtual machine and apply the same vdi hard disk to it but the same problem occurs. This tells me that something bad changed in the vdi file, or?

I'm able to assign an optical drive, e.g. a ubuntu live disc, and VirtualBox starts it normally without any problem.
Using a Ubuntu Live CD within VirtualBox I'm still able to access the partition and all of the data which means I can rescue that in that way. So, the system is still there, it is not deleted, but it cannot be started!
However, I don't want to do it in that way, since I think VirtualBox destroyed the grub menu.lst or any other configuration in the boot chain which finally prevents starting ubuntu correctly!
Currently I'm searching and trying to create the origin assignments within grub so that the right still existing kernel and so on can be triggered and the ubuntu system can be started again normally.
existing-kernel.png


There is a strange thing I discovered so far: the grub.cfg contains of this defect ubuntu (14.04) system contains non-UTF-8 content (checked witzh unix command less). I checked the grub.cfg of another working ubuntu (16.04) using same VirtualBox and the grub.cfg contains human-readable UTF-8 content...

1. is the grub.cfg file corrupted?
2. is maybe the menu-lst file missing or corrupted (how can I check this)?

Please help!

[oldfred]

Moved to virtualization sub-forum.

Do not know virtual installs.

Grub has not used menu.lst since grub legacy back in 2009.

With grub2 you have a grub.cfg which is the menu you see and some settings.
If old BIOS configuration, not newer UEFI:
Normally grub is in MBR and that starts boot process, and then part of grub, core.img which is in the sectors after the MBR find and load grub.cfg.

Can you run Boot-Repair on your virtual install?

May be best to see details, you can run from your Ubuntu live installer or any working install, use ppa version not older Boot-Repair ISO:
Post the link to the Create BootInfo summary report. Is part of Boot-Repair:
https://help.ubuntu.com/community/Boot-Info and:
https://sourceforge.net/p/boot-repair/home/Home/

[lesscaffee]

Thank you very much for your reply!

I will try out next boot-repair, since I was not totally successful with editing the possibly corrupted grub.cfg file (in /media/ubuntu/<id_of_megabyte-big_partition>/grub/grub.cfg) with human-readable content. The result of it:

• the grub menu is loaded now and lists the kernel options I had edited in the grub.cfg file:
  2018-01-05_17h54_58.png
  I don't know what 'serial console 115200' means and if that has any effect??
  
  2018-01-05_17h52_24.png
• memtest86 runs failure-free
• however when selecting ubuntu to start, the following errors appear:
  
  2018-01-05_17h52_12.png

This is the what I edited in the grub.cfg file after I had retrieved in the live-cd ubuntu information about logic volumes and groups with (sudo vgdisplay and sudo lvdisplay):

## 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 $prefix/grubenv ]; then
  set have_grubenv=true
  load_env
fi
if [ "${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 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$feature_default_font_path = xy ] ; then
   font=unicode
else
insmod part_msdos
insmod lvm
insmod ext2
set root='lvmid/cjhDLV-enje-9nc3-1YvI-0AMT-sbid-Q30obB/QhHQq7-l4is-gBkf-eb4i-cKFp-ZeRF-i1rpzP'
if [ x$feature_platform_search_hint = xy ]; then
  search --no-floppy --fs-uuid --set=root --hint='lvmid/cjhDLV-enje-9nc3-1YvI-0AMT-sbid-Q30obB/QhHQq7-l4is-gBkf-eb4i-cKFp-ZeRF-i1rpzP'  a354d856-8285-4612-921c-3b2f99c6c40b
else
  search --no-floppy --fs-uuid --set=root a354d856-8285-4612-921c-3b2f99c6c40b
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_US
  insmod gettext
fi
terminal_output gfxterm
if [ "${recordfail}" = 1 ] ; then
  set timeout=30
else
  if [ x$feature_timeout_style = xy ] ; then
    set timeout_style=hidden
    set timeout=0
  # Fallback hidden-timeout code in case the timeout_style feature is
  # unavailable.
  elif sleep --interruptible 0 ; then
    set timeout=0
  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
if background_color 44,0,30,0; then
  clear
fi
### 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-a354d856-8285-4612-921c-3b2f99c6c40b' {
    recordfail
    load_video
    gfxmode $linux_gfx_mode
    insmod gzio
    if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
    insmod part_msdos
    insmod ext2
    set root='hd0,msdos1'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  6f4539bb-ef7a-444f-a1b6-742d5c79798c
    else
      search --no-floppy --fs-uuid --set=root 6f4539bb-ef7a-444f-a1b6-742d5c79798c
    fi
        linux    /vmlinuz-3.13.0-137-generic root=/dev/mapper/ubuntu--vg-root ro  quiet splash $vt_handoff
    initrd    /initrd.img-3.13.0-137-generic
}
submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-a354d856-8285-4612-921c-3b2f99c6c40b' {
    menuentry 'Ubuntu, with Linux 3.13.0-137-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.13.0-137-generic-advanced-a354d856-8285-4612-921c-3b2f99c6c40b' {
        recordfail
        load_video
        gfxmode $linux_gfx_mode
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_msdos
        insmod ext2
        set root='hd0,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  6f4539bb-ef7a-444f-a1b6-742d5c79798c
        else
          search --no-floppy --fs-uuid --set=root 6f4539bb-ef7a-444f-a1b6-742d5c79798c
        fi
        echo    'Loading Linux 3.13.0-137-generic ...'
            linux    /vmlinuz-3.13.0-137-generic root=/dev/mapper/ubuntu--vg-root ro  quiet splash $vt_handoff
        echo    'Loading initial ramdisk ...'
        initrd    /initrd.img-3.13.0-137-generic
    }
    menuentry 'Ubuntu, with Linux 3.13.0-137-generic (upstart)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.13.0-137-generic-init-upstart-a354d856-8285-4612-921c-3b2f99c6c40b' {
        recordfail
        load_video
        gfxmode $linux_gfx_mode
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_msdos
        insmod ext2
        set root='hd0,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  6f4539bb-ef7a-444f-a1b6-742d5c79798c
        else
          search --no-floppy --fs-uuid --set=root 6f4539bb-ef7a-444f-a1b6-742d5c79798c
        fi
        echo    'Loading Linux 3.13.0-137-generic ...'
            linux    /vmlinuz-3.13.0-137-generic root=/dev/mapper/ubuntu--vg-root ro  quiet splash $vt_handoff init=/sbin/upstart
        echo    'Loading initial ramdisk ...'
        initrd    /initrd.img-3.13.0-137-generic
    }
    menuentry 'Ubuntu, with Linux 3.13.0-137-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.13.0-137-generic-recovery-a354d856-8285-4612-921c-3b2f99c6c40b' {
        recordfail
        load_video
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_msdos
        insmod ext2
        set root='hd0,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  6f4539bb-ef7a-444f-a1b6-742d5c79798c
        else
          search --no-floppy --fs-uuid --set=root 6f4539bb-ef7a-444f-a1b6-742d5c79798c
        fi
        echo    'Loading Linux 3.13.0-137-generic ...'
            linux    /vmlinuz-3.13.0-137-generic root=/dev/mapper/ubuntu--vg-root ro recovery nomodeset 
        echo    'Loading initial ramdisk ...'
        initrd    /initrd.img-3.13.0-137-generic
    }
}


### 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_memtest86+ ###
menuentry 'Memory test (memtest86+)' {
    insmod part_msdos
    insmod ext2
    set root='hd0,msdos1'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  6f4539bb-ef7a-444f-a1b6-742d5c79798c
    else
      search --no-floppy --fs-uuid --set=root 6f4539bb-ef7a-444f-a1b6-742d5c79798c
    fi
    knetbsd    /memtest86+.elf
}
menuentry 'Memory test (memtest86+, serial console 115200)' {
    insmod part_msdos
    insmod ext2
    set root='hd0,msdos1'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  6f4539bb-ef7a-444f-a1b6-742d5c79798c
    else
      search --no-floppy --fs-uuid --set=root 6f4539bb-ef7a-444f-a1b6-742d5c79798c
    fi
    linux16    /memtest86+.bin console=ttyS0,115200n8
}
### 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 ###
### 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 ###

Since all data is unchanged and accessible and the ubuntu system just gets not started by grub, that virtualbox maybe just corrupted e.g. the grub.cfg file.
My hope is therefore to just restore the right configuration in the grub.cfg file and magically the system starts again.
But so far that doesn't work because of the errors showed above.

[oldfred]

Also do not know LVM type installs.

But bad magic number means you did not preform correct incantations.

Or really issue is versions of grub do not match.
If you chrooted and reinstalled grub to MBR, it may not be version in your install. Or you did not mount both /boot & / (in LVM) when reinstalling grub so it did not correctly update.
If LVM is also encrypted you have to unencrypt as part of grub reinstall or with Boot-Repair.

If you use Boot-Repair's full reinstall of grub in advanced options, it may work.
Boot-Repair is supposed to work with LVM, but make sure both /boot & LVM are mounted.