GPT on BIOS only system

[Snowhog]

So, I Googled and found this: https://superuser.com/questions/1245...245320#1245320

Yes, it’s old, but…

My laptop is BIOS only (purchased before 2015). Can I convert its internal HDD from MBR to GPT?

[oshunluvr]

Yes. GPT is not equivalent to UEFI. UEFI requires GPT. GPT does not require UEFI unless you are booting to MS Windows. That is an operating system requirement, not a BIOS one.

[oshunluvr]

Just to update this thread for others reading here: Discussions on other threads revealed that in some cases, older computers (circa 2015 or earlier) cannot read GPT partitioned drives. One must test or verify before attempting a conversion.

This does not change my stance on UEFI vs. Legacy(BIOS) booting. Too often people conflate UEFI with GPT and they are not the same thing. UEFI (previously just EFI) is a type of boot system as is Legacy aka BIOS booting. One can boot using Legacy booting and still use GPT partitioned disks - with the above possible exception noted. However, in order to use UEFI booting one must use GPT partitioned disks.

One difficulty often encountered when attempting to use BIOS booting on a GPT partitioned disk is part of the GPT disk must be reserved for GRUB2. If one attempts a fresh Legacy install to a GPT disk without this space reserved or available the installer will fail and is not likely to reveal the cause with a useful error message, it will simply fail at the GRUB install step. This is normally encountered when the disk has been already partitioned and the installer will not remove, resize, or otherwise mess with existing partitions for obvious reasons. In my experience if the target drive does NOT have existing partitions, the installer will create the needed BIOS boot partition (1mb in size and of type "EF02") for GRUB and installation will complete. Conversely, If you are installing in UEFI mode to a disk previously containing a UEFI installation, the needed partition for GRUB-EFI already exists and the installer will simply locate and use it.


Fortunately, there is a fairly simple work-around to successfully complete a BIOS install to a GPT partitioned disk with previously existing partitions:

The disk must have a very small BIOS boot partition of type EF02. As previously stated 1mb is the default size, however somewhat smaller than that is sufficient.

There is likely already enough existing free space on your drive without resizing or moving partitions. This is because the default behavior of partitioning tools for quite awhile (years, maybe even a decade) is to start the first partition at sector 2048 to provide correct sector alignment for performance reasons.

This leaves almost a megabyte of space unused at the beginning of your drive - 1007kb to be exact - because the first available sector is 34. Thus sectors 34 to 2047 are unused. You can use this space to provide an EF02 partition for GRUB to use and not affect any other partitions of the drive. The steps are:

1. Open the drive using gdisk
2. Change to the "experts only" menu
3. Set the sector alignment value to 1
4. Return to the main menu
5. Create a new partition beginning at sector 34 and ending at 2047
6. Set the new partition to type EF02
7. Write the partition table to the disk and quit gdisk
8. Begin your legacy installation

Since you are mucking about with a partition table, it is always advisable to have a backup of any important data on the target drive.