Ubuntu – Grub won’t boot after converting MBR partition table to GPT

bootdual-bootgrub2partitioning

I have a problem on booting my Ubuntu 16.04.2 LTS after converting my hard drive from MBR to GPT. As I found it on other websites, I used gdisk to convert the hard drive from MBR to GPT. After doing so and restarted my PC, it turned out that the GRUB OS Menu do not display on boot, instead, the screen just showed me a black screen with a blinking cursor. When I tried to convert the GPT back to MBR partition table, the command seemed to be successful but the GRUB is still not working

Additional Info: I am dual-booting Ubuntu 16.04 and Windows 10

Command used to convert MBT to GPT:

sudo gdisk /dev/sda
Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING PARTITIONS!!

Do you want to proceed? (Y/N): Y

Anyone who knows how to fix the problem? Am I doing something wrong?

Best Answer

Short answer:

grub i386-pc needs a "BIOS boot partition" (i.e. type ef02 in gdisk). So you need to create one after the conversion, before running grub-install again.

Explanation:

It's only natural that grub will not work when youn convert your MBR/msdos partition table to GUID partition table.

The reason is, grub makes use of the "post-MBR gap", that is, the gap between the MBR and the first partition, for embedding a portion of its boot code (a.k.a. core.img). When you convert the partition table to GPT, part of the gap is overwritten with the main GPT, for it lies on LBA 1-33 (where the MBR lies on LBA 0).

Moreover, BIOS grub (a.k.a. grub i386-pc) on GPT no longer make use of the post-MBR, or post-GPT gap for core.img embedding. Instead it needs a "BIOS boot partition" (i.e. type ef02 in gdisk). So you need to create one after the conversion, before running grub-install again.

Also, note that a backup GPT lies at the last 33 logical blocks of the disk. If the last/only partition extended till the last block of the disk, you would need to shrink the filesystem on it to make sure that the filesystem no longer covered the last 33 blocks of the partition/disk, so that the blocks could be safely overwritten with the backup GPT. If you didn't do that before the conversion, there could be some damages done to the filesystem.

Terminology

BIOS = Basic Input/Output System

(U)EFI = (Unified) Extensible Firmware Interface

MBR = Master Boot Record

GPT = GUID Partition Table

UEFI / EFI / BIOS = Firmware interface

MBR / GPT = How the computer knows (per hard disk) what partitions are on the drive and how to boot from them.

UEFI / BIOS

A firmware interface is the way that the firmware (the software inside devices) and operating system interact. It initialises the hardware then runs the operating system and ensures that the operating system drivers can operate the hardware.

The BIOS has been the usual firmware interface that has been used. The UEFI is a newer interface that has several features, such as being faster, having a GUI and being able to start the network card and get an ip address. UEFI replaces EFI. (Those developing EFI saw that there were others doing something similar and so joined them, bringing the ideas of EFI with them. This then became UEFI).

A BIOS requires the bootloader to be at the start of the disk, however a UEFI uses a partition for this and can choose among multiple boot loaders to use.

MBR / GPT + GRUB

The MBR is a section of code at the start of the disk that contains a boot loader (for the BIOS), as well as the partition map and a unique disk identifier.

To install GRUB onto a disk with a MBR, GRUB places a small program in the MBR to load the rest of GRUB from another part of the disk. (This is done because the MBR is too small to contain all of GRUB). The space that is chosen is space between the MBR and the first partition, which usually exists.

GPT is a standard for how the partitons are specified. It does have a 'protective' MBR, however this is only for allowing BIOS based computers to boot and stop tools that only know about MBR from trying to trash GPT. It can have

(How GPT is handled depends on whether it is booting using a BIOS (or UEFI system in BIOS emulation mode) or UEFI. I will focus on UEFI as it relates to the question).

Boot loaders for operating systems are stored in a partion called EFI System Partiton, that is formatted (usually) with FAT32. This is where GRUB is installed.

Converting

First...

We are playing around with the partition table, so guaranteed safety is not possible. It is a risky operation. However, the method should not lose data.

Others coming across this: Do not use on Apple Macs.

Now...

You will need to do this on a live CD (or another linux installation installed to a different disk.)

When dealing with GPT disks, we need to use a GPT aware program. 'GPT fdisk' is a good tool to use and what I will be using. It can be called gptfdisk or gdisk depending on distribution (Ubuntu calles it gdisk). Parted (and Gparted) is also GPT aware, so can 'safely' be used with GPT disks.

To convert you need to:

Resize partitions to fit GPT data and EFI system partition.
Convert the disk and add partitions
Install GRUB to EFI system partition.

1) Resize partition

Use parted (command line) or gparted (GUI) to resize the first and last partition. The first partion should have about 200MiB before it, and the last partition should have 1MiB to 2MiB (either will do) taken off the end.

2) Convert the disk

Run

gdisk /dev/sdx

changing the device you want to convert is /dev/sdx.

It should tell you that it will convert the partiton table.

GPT fdisk (gdisk) version 0.6.14

Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: not present


***************************************************************
Found invalid GPT and valid MBR; converting MBR to GPT format.
THIS OPERATION IS POTENTIALLY DESTRUCTIVE! Exit by typing 'q' if
you don't want to convert your MBR partitions to GPT format!
***************************************************************


Command (? for help):

Now add a new partition, making it of type 'EFI system'. It should find the free space at the beginning (I suggest some low sector number like 34) and automatically use all the free space. The examples use a 4GB USB flash drive with 1 partition already on there, resized as per above.

Command (? for help): n
Partition number (2-128, default 2): 2
First sector (34-7831518, default = 34) or {+-}size{KMGTP}: 
Information: Moved requested sector from 34 to 2048 in
order to align on 2048-sector boundaries.
Use 'l' on the experts' menu to adjust alignment
Last sector (2048-421887, default = 421887) or {+-}size{KMGTP}: 
Current type is 'Linux/Windows data'
Hex code or GUID (L to show codes, Enter = 0700): L
0700 Linux/Windows data    0c01 Microsoft reserved    2700 Windows RE          
4200 Windows LDM data      4201 Windows LDM metadata  7501 IBM GPFS            
7f00 ChromeOS kernel       7f01 ChromeOS root         7f02 ChromeOS reserved   
8200 Linux swap            8301 Linux reserved        8e00 Linux LVM           
a500 FreeBSD disklabel     a501 FreeBSD boot          a502 FreeBSD swap        
a503 FreeBSD UFS           a504 FreeBSD ZFS           a505 FreeBSD Vinum/RAID  
a800 Apple UFS             a901 NetBSD swap           a902 NetBSD FFS          
a903 NetBSD LFS            a904 NetBSD concatenated   a905 NetBSD encrypted    
a906 NetBSD RAID           ab00 Apple boot            af00 Apple HFS/HFS+      
af01 Apple RAID            af02 Apple RAID offline    af03 Apple label         
af04 AppleTV recovery      be00 Solaris boot          bf00 Solaris root        
bf01 Solaris /usr & Mac Z  bf02 Solaris swap          bf03 Solaris backup      
bf04 Solaris /var          bf05 Solaris /home         bf06 Solaris alternate se
bf07 Solaris Reserved 1    bf08 Solaris Reserved 2    bf09 Solaris Reserved 3  
bf0a Solaris Reserved 4    bf0b Solaris Reserved 5    c001 HP-UX data          
c002 HP-UX service         ef00 EFI System            ef01 MBR partition scheme
ef02 BIOS boot partition   fd00 Linux RAID            
Hex code or GUID (L to show codes, Enter = 0700): ef00
Changed type of partition to 'EFI System'

You should now have the EFI partition.

Command (? for help): p
Disk /dev/sdd: 7831552 sectors, 3.7 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 669247F2-37F7-4797-98F9-9CE56F7EA8C8
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 7831518
Partitions will be aligned on 2048-sector boundaries
Total free space is 4029 sectors (2.0 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1          421888         7829503   3.5 GiB     0700  Linux/Windows data
   2            2048          421887   205.0 MiB   EF00  EFI System

Then exit gdisk

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed, possibly destroying your data? (Y/N): y
OK; writing new GUID partition table (GPT).
The operation has completed successfully.

Now use Gparted (or command-line mkfs.vfat -F 32 /dev/partition) to format the partition as FAT32.

3) Install GRUB

This comes with less assurances that the previous part as I have not tried it myself.

I am not sure about this step, so I'll guess using RAOF's instructions:

To switch to grub-efi you'd want to

Find your EFI partition; mount it in /boot/efi. Add this to /etc/fstab¹

Install the grub-efi package

Switch your BIOS boot priority from UEFI and Legacy to UEFI only (or a similar option)

You should work out which version of grub-efi to install with

ioreg -l -p IODeviceTree | grep firmware-abi

If it says EFI32 install the grub-efi-ia32 package, if it says EFI64 install the grub-efi-amd64 package. You can install the packages with

sudo apt-get install <package name>

This will probably only work if you have booted in EFI mode.

If it does not work, you could try these step by step instructions (under "Install GRUB2 in (U)EFI systems") once grub-efi is installed.

Configuring (+ Dual Boot)

If RAOK's instructions work, you should be able to add the following line to /etc/grub.d/40_custom

menuentry "Windows 7" {
        set root='(hd0,gpt1)'
        chainloader /EFI/microsoft/bootmgfw.efi
}

It assumes that Windows is recognised as hd0 by GRUB. It may need to be changed to hd1 in order to work.

Now run

update-grub

to update the config file.

References and further reading

I used several sources.

Rod Smith's "GPT fdisk tutorial". Good for more info.
Wikipedia pages for UEFI, BIOS, MBR and GPT.
Ubuntu's UEFI booting wiki page. Good for more how-to if you get stuck.
UEFI's own about page.

Ubuntu – How to install Ubuntu on a GPT partition table without using EFI boot

GPT partition table is specially used for Hard Disks sizes greater than 2TB.

The partition table is independent of the BIOS mode (i.e Legacy or UEFI).

To create a GPT partition table--

Boot into the live USB or live CD containing Ubuntu.
Click on TRY UBUNTU , so that you can create partition table manually.
Open GParted Partition Editor.
On the top of GParted Menu bar , click on Device --> Create partition Table.
Select GPT from the drop down list and click on Apply.

Now to verify the Partition Table-

Open Terminal
Type the command:

sudo su

(to execute further commands as super user since some files are read-only)

parted -l

(to list all the partitions)

If the above command doesn't work , then try specifying the device as:

parted -l /dev/sda

There you will see that the partition is GPT.

Now install Ubuntu as usual , installation will not rewrite the partition table (partition table remains GPT only) but new partitions are created.