Booting with MBR in UEFI (No CSM mode at all)?

Question

I'm asking here since I can't find a clear and definite answer to if it is possible to boot with an MBR partitioned hard drive in an UEFI-only environment. I've read this question but it is still unclear to me: UEFI with MBR partition table?

I do know CSM mode exists, but the goal here is to only use pure native UEFI since I don't have the CSM mode option in my UEFI. Furthermore, I can't and don't want to convert my hard drive form MBR to GPT, so:

Does Windows Vista, 7, 8, 8.1 and 10 can boot with an MBR in pure native UEFI ?

Does this still applies to Linux ?

Solution 1

As long as you have an EFI system partition, the partition table type doesn't matter. However, Windows (uniquely) requires GPT to use EFI; this is not a problem with Linux.

The following answer gives more details in regards to Windows and it's EFI support.

You're not asking about safety, but about support. Yes, EFI firmware has no trouble with an MBR partition table, which it can cope with just as it can cope with an EFI partition table. You just need to make sure that you have an EFI System Partition.

Your problem is Windows. Microsoft erroneously conflates has a GPT partitioned disc with bootstraps in the EFI way. So your laptop with a modern EFI partition table and modern EFI firmware has been installed and is bootstrapping Windows in the modern EFI way. Change to an MBR style partition table, and Windows will expect to be bootstrapping in the old PC98 way. You'd have to switch on the Compatibility Support Module option in your firmware, if it has it, and either reinstall Windows or individually modify the Microsoft Boot Manager, the system BCD store, the system volume's VBR, and the MBR bootstrap program.

The following answer give more details in regards to Linux and its EFI support.

Linux can certainly boot off an MBR disk in EFI mode. The trouble is that this type of configuration is poorly tested, and you may have problems getting your boot loader registered with the EFI. You might need to name your boot loader EFI/BOOT/bootx64.efi and rely on the EFI using this fallback filename.

Also, most Linux distributions' installation programs won't make it easy to set up this sort of configuration; you may need to install to GPT and convert that to MBR, or install in BIOS mode and then install an EFI boot loader after the fact.

Solution 2

Squircle's answer and various comments are good; however, I want to address a few points....

I don't have the CSM mode option in my UEFI. Furthermore, I can't and don't want to convert my hard drive form MBR to GPT

It is possible to convert from MBR to GPT without losing data. My own GPT fdisk (gdisk) program can do this. I hear that some third-party Windows tools can do it, too, but I don't know the details. Microsoft's own tools cannot do this, though, so do not try to use them for the task. Note that MBR-to-GPT conversion is very simple compared to something like converting from one filesystem to another, so the risk is low -- but not zero. No operation that involves writing to data structures as sensitive as a partition table is zero-risk.

You say you "don't want" to convert to GPT, but you don't say why. If it's simply the hassle or risk of conversion, that's dwarfed by follow-on problems of the near-complete lack of support you'll face by using MBR. (I am, however, assuming that you have or can create adequate backups. If that's incorrect, FIX THAT PROBLEM IMMEDIATELY! Backups are NOT a luxury!) If, like some people, you're comfortable with MBR and just don't see a need to change to some [crotchety old-man voice]new-fangled thingamajig[/crotchty old-man voice], then I recommend you shed your biases. GPT offers real advantages over MBR. Most of them are minor, but if you must boot in EFI mode, the fact that GPT is the standard for EFI-mode booting is not a minor advantage; it's a major one.

Windows (uniquely) requires GPT to use EFI

My understanding is that this is a requirement of the Windows installer, but it's possible to get Windows to boot in EFI mode from an MBR disk by using various tricks, like converting from GPT to MBR after installing the OS, or by replacing a BIOS-mode boot loader with an EFI-mode boot loader without changing the partition table type. That said, my knowledge of precisely how best to achieve this goal is foggy, since I've only read about it; I've never done it myself. I've read about this only once or twice in several years, so this is not something that's done very often.

Can the EFI system partition be a logical partition ?

In theory, it shouldn't matter. In practice, I don't know. Booting in EFI mode from an MBR disk is such a corner case that there's a small but non-negligible chance that literally nobody in the world has tried using an MBR logical partition as an ESP. Certainly I haven't tried it. Please keep this in mind if you decide to pursue booting in EFI mode from an MBR disk -- your odds of getting expert (or even advanced amateur) help drop to near zero should you run into problems.

Overall, my recommendation is to look into MBR-to-GPT conversion tools. Chances are you'll be able to convert from MBR to GPT without trouble. The result will be less likely to cause problems than trying to boot in EFI mode from an MBR disk. You should, however, back up before doing this. That would be my advice even without an MBR-to-GPT conversion in the picture, though; mucking with boot loaders can be as risky as mucking with partition tables, so trying that without a backup is inadvisable, too.

Comments

[Ramhound]

Windows does not support UEFI if the partition schema is MBR. Windows requires legacy mode to be enabled if the partition schema is MBR. Windows Vista does not support UEFI mode at all. Only the 64-bit version Windows 7 supports UEFI. Windows 8+ are more forgiving but the same base restrictions dealing with MBR/GPT still apply.

[squircle]

Possible duplicate of UEFI with MBR partition table?

[X.LINK]

I've read this answer before posting, but it is still very unclear for me. What about Linux ?

[Ramhound]

@X.LINK - There is already an answer that explain that also.

[Ramhound]

I quoted the original answer you linked to. I also on your behalf added information about Linux to your answer. Otherwise the answer seemed incomplete without it.

[X.LINK]

The answer is clearer now (especially the first part:)). One last question: Can the EFI system partition be a logical partition ?

[Ramhound]

@X.LINK - What does your research indicate?

[X.LINK]

I only found information for ESP in GPT disks but nothing about being able to be a Logical Partition in an MBR based disk.

[squircle]

@Ramhound Thanks -- my original answer was a bit rushed.