Hybrid partition table

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Most x86 and x86_64 systems only use either the MBR (MS-DOS based) partition table layout, or the GPT (GUID Partition Table) layout. However, in certain situations it might be necessary for a GPT partition layout to have an "MBR view" overlayed. This is called Hybrid MBR/GPT.

Using hybrid partition table

Using a hybrid partition table might be necessary when dual-booting operating systems that are blind to a GPT-partitioned disk. In these situations, the partition definition requires a tricky workaround.

Creating with gdisk

Creating a hybrid MBR is easy using gdisk: at the main menu press the r key (for recovery and transformation), then press h key (CHS recompute), then enter the series of partitions (less or equal to 3) in the order they should be be listed in the MBR. Finally enter the hexadecimal partition type without the 0x prefix or accept the defaults by repeatedly pressing the y key.

Enable only a single boot flag as found the following example:

root #gdisk /dev/sdg
GPT fdisk (gdisk) version 0.8.1
 
Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: present
 
Found valid MBR and GPT. Which do you want to use?
 1 - MBR
 2 - GPT
 3 - Create blank GPT
 
Your answer: 2
root #gdisk /dev/sdg
...
Using GPT and creating fresh protective MBR.
 
Command (? for help): r
Recovery/transformation command (? for help): p
Disk /dev/sdg: 625142448 sectors, 298.1 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 744E8BF3-39A4-4908-8646-AC2E5661C8CF
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 625142414
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)
 
Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048       566233087   270.0 GiB   8300  Linux filesystem
   2       566233088       625142414   28.1 GiB    0700  Microsoft basic data
Recovery/transformation command (? for help): h
 
WARNING! Hybrid MBRs are flaky and dangerous! If you decide not to use one,
just hit the Enter key at the below prompt and your MBR partition table will
be untouched.
 
Type from one to three GPT partition numbers, separated by spaces, to be
added to the hybrid MBR, in sequence: 2 1
Place EFI GPT (0xEE) partition first in MBR (good for GRUB)? (Y/N): N
 
 
Creating entry for GPT partition #2 (MBR partition #1)
Enter an MBR hex code (default 07): 
Set the bootable flag? (Y/N): Y
 
Creating entry for GPT partition #1 (MBR partition #2)
Enter an MBR hex code (default 83): 
...
Unused partition space(s) found. Use one to protect more partitions? (Y/N): N
 
Recovery/transformation command (? for help): o
 
Disk size is 625142448 sectors (298.1 GiB)
MBR disk identifier: 0x00000000
MBR partitions:
 
Number  Boot  Start Sector   End Sector   Status      Code
   1      *      566233088    625142414   primary     0x07
   2                  2048    566233087   primary     0x83
   4                     1         2047   primary     0xEE
 
 
Recovery/transformation command (? for help): w
Note
In the example above, N was used to answer the safe questions. Without N, fdisk -l /dev/sdg will list the MBR partition list just fine but ls /dev wouldn't list the partitions - and even more annoying: the partitions wouldn't be listed in DOS/BIOS.

Supporting boot loaders

GRUB2

GRUB2 supports booting from GPT and MBR partition tables via the part_gpt and part_msdos modules. However, a successful setup and GRUB2's ability to boot correctly depends both on loaded partition modules and on correctly installing GRUB2. A determining factor in how GRUB2 is installed (BIOS-MBR vs BIOS-GPT) is the presence of a EE00 (GPT protective) partition as the first partition in the MBR.

With a hybrid MBR, GRUB2 will install if the first partition in the MBR table is an EE00 partition. An EF02 partition should be present anywhere in the disk and not necessarily in the MBR list. Leave enough open sectors before the first partition!

Note
In this case one does not have to add/list an EE00 partition in the hybrid MBR list manually, gdisk will provide a prompt to list one if booting with GRUB2 before displaying the provided list (up to 3 partitions) by the user.

GRUB2 requires an EE00 partition type at the first listing in the MBR partition list entry, however it may pose a problem when booting other operating systems on the same disk (especially Windows). A workaround is available in order to recognize a GPT partitioned disk with (or without) hybrid MBR. Removing any protective EE00 partition with the fdisk command.

Warning
Do not use gdisk to remove the EE00 partition! The gdisk command does not work because a "fake" partition exists to protect the main GPT table while booted in BIOS - MBR mode.

Use other disk tools such as parted (sys-block/parted) or gparted (sys-block/gparted) to resize after that. Otherwise use gdisk to add protective EE00 partitions to protect at least GPT main and backup the partition before using any non-GPT aware tools if necessary.

Another issue should be addressed when attempting to use a GPT Hybrid MBR configuration: Windows MSDOS based bootloaders) may not find suitable partitions to boot from if there is a EE00 partition protecting the GPT main partition table (sectors 1-2047). If this happens remove the protective EE00 partition before getting a valid MBR partition list in [MS]DOS tools (to format a partition for example, the tools recognized a GPT partitioned disk at least). And then use gdisk to recompute the CHS value for hybrid MBR (in the recovery and transformation menu, or in the expert section of gdisk's main menu).