Why does first grub boot menu entry result in kernel panic, while the rescue boot menu entry results in a normal startup?

edit

I have performed a /boot/efi installation of Fedora 21 beta to hd0 with LVM partitions and it is updating and operating fine. I wish to make use of btrfs and its raid 1 functionality. To that end I have installed the released Fedora 21 to hd1 a couple of times with /boot/efi. Each time, when I reboot the system and select the first menuitem I get:

kernel panic not syncing vfs unable to mount root fs on unknown-block(8,19)

If I select the second menu item...

Fedora, with Linux 0-rescue-4e5b2ea7618d49ee94bac4cbb1876d81

...the system starts up as a normal system and works fine.

Any ideas about what could cause this?

Can I fix it easily after the install?

Here is the output of os-prober:

[root@<nameremoved> ~]# os-prober
/dev/sdb3:Fedora release 21 (Twenty One):Fedora:linux:btrfs:UUID=485f133f-9e8e-4d94-8336-00c319dceae6:subvol=root

Below is the contents of grub.cfg in /boot/efi/EFI/fedora

### BEGIN /etc/grub.d/00_header ###
set pager=1

if [ -s $prefix/grubenv ]; then
  load_env
fi
if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   set boot_once=true
else
   set default="${saved_entry}"
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 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
}

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

### BEGIN /etc/grub.d/10_linux ###
menuentry 'Fedora, with Linux 3.17.4-301.fc21.x86_64' --class fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.17.4-301.fc21.x86_64-advanced-485f133f-9e8e-4d94-8336-00c319dceae6' {
    load_video
    set gfxpayload=keep
    insmod gzio
    insmod part_gpt
    insmod btrfs
    set root='hd1,gpt3'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd1,gpt3 --hint-efi=hd1,gpt3 --hint-baremetal=ahci1,gpt3  485f133f-9e8e-4d94-8336-00c319dceae6
    else
      search --no-floppy --fs-uuid --set=root 485f133f-9e8e-4d94-8336-00c319dceae6
    fi
    linuxefi /root/boot/vmlinuz-3.17.4-301.fc21.x86_64 root=/dev/sdb3 ro rootflags=subvol=root rhgb quiet
}
menuentry 'Fedora, with Linux 0-rescue-4e5b2ea7618d49ee94bac4cbb1876d81' --class fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-4e5b2ea7618d49ee94bac4cbb1876d81-advanced-485f133f-9e8e-4d94-8336-00c319dceae6' {
    load_video
    insmod gzio
    insmod part_gpt
    insmod btrfs
    set root='hd1,gpt3'
    if [ x$feature_platform_search_hint = xy ]; then
      search --no-floppy --fs-uuid --set=root --hint-bios=hd1,gpt3 --hint-efi=hd1,gpt3 --hint-baremetal=ahci1,gpt3  485f133f-9e8e-4d94-8336-00c319dceae6
    else
      search --no-floppy --fs-uuid --set=root 485f133f-9e8e-4d94-8336-00c319dceae6
    fi
    linuxefi /root/boot/vmlinuz-0-rescue-4e5b2ea7618d49ee94bac4cbb1876d81 root=UUID=485f133f-9e8e-4d94-8336-00c319dceae6 ro rootflags=subvol=root rhgb quiet
    initrdefi /root/boot/initramfs-0-rescue-4e5b2ea7618d49ee94bac4cbb1876d81.img
}

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

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