Does the 'mount' command rely on MBR to detect partitions, or can it use the GPT partition table?

Question

TL:DR

I have Mac, which means I have EFI and a GPT formatted disk with a hybrid MBR. I am considering adding a fifth partition; to do this, I will mask my first three GPT partitions with one partition in my protective and hybrid MBR.

However, if, say, I fire up an operating system after these (say, on my fourth I have Linux), will I be able to mount filesystems from inside the protective MBR? I.e., can an OS like Linux that has been booted from a legacy MBR view GPT partitions, or does it rely on the MBR to tell it what partitions exist?

Does mount rely on MBR, or will it find (my) GPT disks?

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Full / Old Question

I have a hybrid MBR of 4 partitions on my Mac.

1. EFI System Partition
2. Mac OS X
3. Mac Recovery Partition
4. Ubuntu

Now, I want to add Archlinux to the end, i.e., append 5. Archlinux, but I would also like to maintain a reasonable MBR (because an MBR can only have four records).

My idea is to create a MBR partition that covers both partitions 1 and 2, or 2 and 3, or 1-3, thus making the MBR think that there is one partition, and nothing will touch it (this is typical of GPT disks, which have a protective MBR which covers all the GPT partitions with one legacy MBR partition for safety reasons).

My question is: Does Ubuntu use the MBR to mount filesystems? If I do create my nice protective partition, will I no longer be able to mount my Mac OS X filesystem in Ubuntu, assuming Ubuntu is installed using the legacy MBR method, and not the GPT / EFI boot Ubuntu.*

*Or maybe EFI compatibility in Ubuntu is better now?

Answer 1

The answer is OS-dependent. It's not really mount per se that uses GPT or MBR; it's the OS kernel. OS X's kernel depends on the GPT data in preference to the MBR data, so it reads hybrid MBR disks as GPT disks. The same is true of the Linux kernel if that kernel includes GPT support, as most binary kernels distributed by major distributions do. If you build your own kernel and omit GPT support, though, it will ignore the GPT partitions and use the MBR partitions instead.

Windows ignores the GPT data if it sees a hybrid MBR. That's what makes hybrid MBRs useful (albeit ugly and dangerous): Windows sees a hybrid MBR disk as an MBR disk, which means it will install to the disk when booted in BIOS mode, whereas Windows will not install to a GPT disk when booted in BIOS mode.

FreeBSD works in a third way: It presents both sets of partitions using different device nodes. GRUB 2 does something similar; you can specify which table to use by preceding the partition number with a code, as in (hd0,gpt3) to use the third GPT partition on the first disk.

One more point: Because of the dangers of hybrid MBRs, if you're dual-booting between OS X and Linux but not booting Windows, it's best to convert your hybrid MBR to a standard protective MBR. Given the Mac's architecture, though, this will require you to boot Linux in EFI mode. This works fine on most Macs, but there are problems with hardware detection on some models. Thus, you may want to install an EFI-capable boot loader to test your ability to boot and use Linux in EFI mode before you convert your hybrid MBR to a protective MBR. You can read more about EFI boot loaders for Linux here. You can convert a hybrid MBR to a conventional protective MBR by using GPT fdisk (gdisk); type x to enter the experts' menu, type n to create a new protective MBR, and then type w to save your changes.