Can I use the full capacity of an external storage drive larger than 2TB with legacy BIOS?

Question

I have an Acer C720P Chromebook running Gallium OS. I'm fairly new to Linux so this is my first time navigating hardware compatibility issues. I'm interested in configuring the machine as a Plex server, but being a Chromebook, it only has something like 32GB internal storage.

I've learned that my machine contains SeaBIOS, which is legacy-only and doesn't support UEFI. But drives >2TB need to be formatted in GPT which requires UEFI for the BIOS to read all sectors/blocks. At least, I believe this is accurate. But the posts where I read about this were focused on booting from the drive. I'm not doing that-- I want to boot from the internal 32GB drive and use a large >2TB external storage drive.

What I don't know is whether BIOS (and UEFI) are involved in Gallium being able to read/manage my drive when I plug it in. If they are, I can't use that big a drive. And if they aren't, are there any compatibility issues I ought to watch out for when purchasing a storage drive? Thanks!!

Answer 1

What you've learned is partially accurate, but it's a mix of two (or three) different limits:

1. The MBR partition table format cannot describe partitions beyond 2TiB (or more precisely, beyond a certain number of sectors), as it only has 32 bits of space for the sector count. The GPT partition table has no such limit.

   This limit remains relevant regardless of the OS and regardless of boot mode, as it affects the on-disk metadata. (The OS can access the entire disk in "raw" mode, you just cannot create volumes that go beyond the boundary.)

   Fortunately, for a data disk, it's very simple to just repartition it using GPT – and any large disk will most likely come GPT-partitioned straight out of the box.

2. As far as "legacy" BIOS is involved, a bootloader that uses BIOS-provided "read/write disk" operations usually cannot access beyond 2TiB, again because the BIOS calls only deal with 32-bit values.

   This limit is only relevant to the system disk, and it does not affect UEFI boot, only legacy/CSM mode.

   This limit stops being relevant once the OS has successfully booted, as the OS does not call into the firmware for accessing the disk (that would be quite slow) – it always directly talks to the disk controller.

   All modern versions of Windows and Linux, and probably every other operating system, are able to access disks of any size (even if it's the system disk).

3. When the "legacy" BIOS is in use, the Windows system disk must use MBR, and when UEFI is in use, the system disk must use GPT.

   This effectively restricts the Windows system disk to only 2TiB usable if the OS is booting in BIOS mode.

   It has no effect on "data" disks; they can freely use GPT regardless of how the system has booted.

   (Linux doesn't have restrictions on the combinations in general, and could boot from a GPT disk in legacy BIOS mode – in that case only the bootloader must be entirely within the 2TiB range, but once the Linux kernel is running the full capacity of the system disk becomes available.)

So in short, when you're booting in legacy BIOS mode, your system disk needs to be 2TiB or less, but data disks have no such restrictions.

Answer 2

But drives >2TB need to be formatted in GPT which requires UEFI for the BIOS to read all sectors/blocks. At least, I believe this is accurate.

No, it's completely wrong.

You're confusing Windows requirements for installation in UEFI or BIOS (Legacy/CSM) modes with something where such limitations aren't applicable.

This requirements only apply to (1) Windows and (2) Windows' system drive. External drives with GPT can and were used with old BIOS systems irrespective of the OS.