I've had this issue before when trying out Linux Mint, but I was unable to get any assistance. I was then lead to believe that it was a problem related to Mint, and not grub.

I installed Ubuntu 12.04 tonight on a second partition alongside Windows 7. I installed from a USB stick, and everything went peachy until I rebooted without the stick in my tower. It now says:

error: no such device: 20cec6ca-4024-4237-84c3-2dba3c851497
grub rescue >

I've verified via ls -l /dev/disk/by-uuid that my drive where Ubuntu is installed matches the UUID that supposedly doesn't exist. The UUID of my thumb drive when inserted happens to be 06B3-9C68.

There is no mention of my USB drive's UUID anywhere in /boot/grub/grub.cfg

I've also tried to re-install GRUB after booting into my system, removing the stick, and running grub-install /dev/sda. It still happens, and I cannot boot without the USB drive inserted into the computer. And what really gets my goat is that the boot order of my system is CDROM>Hard Drive>USB. It's not even reaching the USB to try to boot from it, so why does it matter that it's not there?

Edit: Also, I ran grub-config without the stick in followed by another grub-install. Still no go.

FWIW, here's my grub.cfg file:

#
# 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 $prefix/grubenv ]; then
  set have_grubenv=true
  load_env
fi
set default="0"
if [ "${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 {
  insmod vbe
  insmod vga
  insmod video_bochs
  insmod video_cirrus
}

insmod part_msdos
insmod ext2
set root='(hd1,msdos5)'
search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
if loadfont /usr/share/grub/unicode.pf2 ; then
  set gfxmode=auto
  load_video
  insmod gfxterm
  insmod part_msdos
  insmod ext2
  set root='(hd1,msdos5)'
  search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
  set locale_dir=($root)/boot/grub/locale
  set lang=en_CA
  insmod gettext
fi
terminal_output gfxterm
if [ "${recordfail}" = 1 ]; then
  set timeout=-1
else
  set timeout=10
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; 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 [ ${match} = 0 ]; then
    set linux_gfx_mode=keep
      else
    set linux_gfx_mode=text
      fi
    else
      set linux_gfx_mode=text
    fi
  else
    set linux_gfx_mode=keep
  fi
else
  set linux_gfx_mode=text
fi
export linux_gfx_mode
if [ "$linux_gfx_mode" != "text" ]; then load_video; fi
menuentry 'Ubuntu, with Linux 3.2.0-25-generic-pae' --class ubuntu --class gnu-linux --class gnu --class os {
    recordfail
    gfxmode $linux_gfx_mode
    insmod gzio
    insmod part_msdos
    insmod ext2
    set root='(hd1,msdos5)'
    search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
    linux   /boot/vmlinuz-3.2.0-25-generic-pae root=UUID=20cec6ca-4024-4237-84c3-d2ba3c851497 ro   quiet splash $vt_handoff
    initrd  /boot/initrd.img-3.2.0-25-generic-pae
}
menuentry 'Ubuntu, with Linux 3.2.0-25-generic-pae (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os {
    recordfail
    insmod gzio
    insmod part_msdos
    insmod ext2
    set root='(hd1,msdos5)'
    search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
    echo    'Loading Linux 3.2.0-25-generic-pae ...'
    linux   /boot/vmlinuz-3.2.0-25-generic-pae root=UUID=20cec6ca-4024-4237-84c3-d2ba3c851497 ro recovery nomodeset 
    echo    'Loading initial ramdisk ...'
    initrd  /boot/initrd.img-3.2.0-25-generic-pae
}
submenu "Previous Linux versions" {
menuentry 'Ubuntu, with Linux 3.2.0-23-generic-pae' --class ubuntu --class gnu-linux --class gnu --class os {
    recordfail
    gfxmode $linux_gfx_mode
    insmod gzio
    insmod part_msdos
    insmod ext2
    set root='(hd1,msdos5)'
    search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
    linux   /boot/vmlinuz-3.2.0-23-generic-pae root=UUID=20cec6ca-4024-4237-84c3-d2ba3c851497 ro   quiet splash $vt_handoff
    initrd  /boot/initrd.img-3.2.0-23-generic-pae
}
menuentry 'Ubuntu, with Linux 3.2.0-23-generic-pae (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os {
    recordfail
    insmod gzio
    insmod part_msdos
    insmod ext2
    set root='(hd1,msdos5)'
    search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
    echo    'Loading Linux 3.2.0-23-generic-pae ...'
    linux   /boot/vmlinuz-3.2.0-23-generic-pae root=UUID=20cec6ca-4024-4237-84c3-d2ba3c851497 ro recovery nomodeset 
    echo    'Loading initial ramdisk ...'
    initrd  /boot/initrd.img-3.2.0-23-generic-pae
}
}
### 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='(hd1,msdos5)'
    search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
    linux16 /boot/memtest86+.bin
}
menuentry "Memory test (memtest86+, serial console 115200)" {
    insmod part_msdos
    insmod ext2
    set root='(hd1,msdos5)'
    search --no-floppy --fs-uuid --set=root 20cec6ca-4024-4237-84c3-d2ba3c851497
    linux16 /boot/memtest86+.bin console=ttyS0,115200n8
}
### END /etc/grub.d/20_memtest86+ ###

### BEGIN /etc/grub.d/30_os-prober ###
menuentry "Windows 7 (loader) (on /dev/sda1)" --class windows --class os {
    insmod part_msdos
    insmod ntfs
    set root='(hd0,msdos1)'
    search --no-floppy --fs-uuid --set=root 9014706714705268
    chainloader +1
}
### 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  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###