Legacy BIOS Issues with GPT

by Rod Smith, rodsmith@rodsbooks.com

Last Web page update: 2/22/2014, referencing GPT fdisk version 0.8.10

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Note: This page is part of the documentation for my GPT fdisk program.

Today, two types of firmware are common on tablet, notebook, desktop, and server computers: the older Basic Input/Output System (BIOS) and the newer Extensible Firmware Interface (EFI) or its variant, the Unified EFI (UEFI), which is essentially EFI 2.x. GPT was created as part of the EFI specification, so of course GPT works well with EFI-based computers. There's no law that says that GPT can't be used with other computers, though. In particular, BIOS is so simple that it doesn't understand the partition table type used on the disk—it's the role of the boot loader and subsequent software to interpret the partition table. Thus, in theory there should be no problem using GPT with BIOS-based computers.

Unfortunately, in practice, there are occasional BIOS/GPT quirks or even downright incompatibilities. Furthermore, many modern computers with EFIs provide a Compatibility Support Module (CSM), which enables them to boot using BIOS-mode boot loaders. The trouble is that the firmware may use the partition table type as a cue for what type of boot mode to use, thus causing problems when trying to boot in BIOS mode from a GPT disk. In either case, a computer with a GPT disk may not boot or will boot very slowly because the BIOS power-on self-test (POST) takes an inordinately long time to finish. I've seen such reports in online forums and public bug-report systems, several users have e-mailed me with such reports, and I've encountered it at least once, and maybe twice, myself.

Causes of Incompatibility

This behavior is quite peculiar, since as far as I know the BIOS should not attempt to read the disks, beyond verifying their presence and reading the boot code in the MBR. There are two specific bugs I'm aware of that cause this type of problem, but there may be others:

There may be other problems, too; many reports are vague, and my own ability to investigate is limited to the computers I own. It's conceivable that some boot loaders could become confused by GPT, so a GPT-incompatible boot loader could cause boot problems. My subjective impression is that most of the problems occur on EFI-based computers set to boot in BIOS/CSM/legacy mode. Thus, the problem is most common in computers purchased since 2011; however, some older computers (particularly some Intel motherboards) used EFIs that exhibited these symptoms well before that date. Such problems can usually be corrected as described below. Even when I've been in physical possession of a board with a problem, investigations have proved difficult. In one case (a system with a dual-core AMD CPU and an AMI BIOS), booting was sluggish, but sped up when I made a random change to the GPT. To this day I'm not even sure if this was a GPT issue or something else. My attempts to re-create the problem have been unsuccessful.

Broadly speaking, there are two possible ways to work around such problems: Stick with the BIOS or abandon BIOS-style booting.

Sticking with the BIOS

There are several possible remedies when booting a GPT disk on a BIOS-based computer. If your computer is completely unresponsive, you'll have to disconnect your hard disk from the computer and connect it in some other way or to another computer. I've heard of people successfully hot-plugging SATA drives after booting without the affected disk, but this is very risky with PATA drives. External enclosures or "naked" adapters to plug PATA or SATA disks into USB ports are good options. I recommend you try the following, in more-or-less the listed order, should you encounter such problems (but see below for an entirely different type of possible solution):

  1. Set the boot flag on the type-0xEE partition in the protective MBR. You can do this with several different programs:
  2. Use the h option on the gdisk experts' menu, or the equivalent -C or --recompute-chs option to sgdisk, to recompute the CHS values for the entries in the protective MBR. This approach works around a bug I've encountered in at least one BIOS, as described earlier. This gdisk option recomputes these values to be more to the BIOS's liking, although it violates the GPT specification.
  3. Ensure that your GPT disk does not have an EFI System Partition (ESP), which is where an EFI stores its boot loaders. Some EFIs use the presence of an ESP as a clue to favor EFI-mode booting, and so such a partition can interfere with BIOS/CSM/legacy-mode booting. Of course, removing the ESP is not an option if you must dual-boot, with one OS in EFI mode and another in BIOS/CSM/legacy mode. Such configurations are inadvisable in most cases. If you really must boot in this way, look into my rEFInd boot manager, which is an EFI boot loader that supports chainloading to BIOS/CSM/legacy boot loaders.
  4. Install a second disk that uses an MBR partition table, preferably with a partition marked as active/bootable. Even a USB flash drive might do the trick.
  5. Use Linux fdisk or a similar tool to delete the EFI GPT protective partition and re-create it. If you use Linux fdisk for this, you should first type c and u to enable entry of sector values as low as 1. Unfortunately, fdisk versions after 2.17 set the minimum sector value to 2048, so you'll need to use an older version of fdisk. This procedure creates a protective MBR that should be identical to the one created by approach #2; however, you can tweak it in various ways if necessary.
  6. Create a hybrid MBR configuration. I recommend creating a hybrid MBR with a large 0xEE (EFI GPT) partition at the start, with just the last GPT partition in the MBR—unless of course you need a hybrid MBR with which to boot Windows or some other GPT-unfriendly OS. Note that hybrid MBRs, at least as generated by GPT fdisk, contain CHS values that a BIOS may find more acceptable than the ones used in technically correct protective partitions, so this option can be very similar to option #2 in many ways.
  7. If the problem occurred on a non-boot disk or on a disk that you've just converted from MBR format, try blanking out the boot loader code at the start of the MBR. You can do this in Linux with dd, as in dd if=/dev/zero of=/dev/sda bs=440 count=1 to blank out the boot loader on /dev/sda. Note the unusual 440 block size (bs) option; this is the length of the boot loader code in the MBR. Using a larger value (particularly anything larger than 446) will begin intruding on MBR data structures. Be sure to include both the bs and count options! If you omit them, and especially the count option, you'll erase data well beyond your MBR!
  8. If the problem occurred on a boot disk that you've just converted from MBR to GPT format, re-install your boot loader, or install a new boot loader. This may entail two installations, or a wipe and a re-installation, since the computer may need to boot with the disk in its normal position before you'll be able to install the boot loader normally.
  9. If you've just converted from MBR to GPT, reverse that process. You can do this with gdisk, as described here.
  10. Disconnect the disk, enter the firmware setup utility, and adjust any options that seem relevant to the boot process. For instance, you might change the boot order. It's conceivable that this will at least enable you to boot an emergency CD with your hard disk connected. You may be able to boot from GRUB placed on a USB flash drive, too.
  11. Upgrade your firmware (BIOS or EFI). Most of these problem are caused by firmware bugs, and an updated firmware might conceivably fix the problem. In fact, I found one reference (for a Tyan Tomcat K8E-SLI motherboard) to a BIOS upgrade that fixes a GPT incompatibility bug. (Note that most manufacturers refer to their EFIs as BIOSes, so you may need to look for a "BIOS upgrade" even if you know you've got an EFI.)

I can't guarantee that any of these actions will do any good on any particular system. These problems are rare and are not as yet well documented, at least not that I've found, so some of these suggestions are speculative.

Abandoning BIOS Booting

An entirely different class of solution is possible with some boards: Abandon BIOS booting in favor of EFI or UEFI booting. Until the spring of 2011, most motherboards did not support EFI booting, so this option may not even be available to you. Beginning in the spring and summer of 2011, though, most new computers began to include EFI firmware, although most didn't advertise that fact initially and instead relied on their CSMs for BIOS-mode booting by default. The vast majority of computers sold with Windows 8 are also EFI-enabled, and in fact they ship configured to boot Windows in EFI mode. If an EFI option is available, then in EFI mode, the motherboard handles GPT partitions just fine, since GPT is part of the EFI specification. This method is easiest to implement with a fresh OS installation—but only if the OS provides good EFI installation support. Fedora 14 and later, Ubuntu 11.04 and later, and OpenSUSE 11.4 and later all work (and some earlier versions may, too). As a general rule, Linux distributions' EFI support has been improving rapidly, so using the latest version available is worthwhile if you're planning an EFI-mode boot. For more information on the topic of EFI-mode booting, see my Managing EFI Boot Loaders for Linux and Linux on UEFI: A Quick Installation Guide pages. Note also that I maintain the rEFInd boot manager, which you may want to use to enable EFI-mode booting of Linux.

If you've got an existing OS installation, switching to (U)EFI booting will require changing your boot loader configuration. This is easy to do for Linux, provided you understand the needs and quirks of your (U)EFI-capable boot loader. (See the "Booting from GPT" page of this document or my EFI Boot Loaders for Linux page for some comments on this issue.)

(U)EFI installations work best when the OS architecture is matched to the (U)EFI architecture. For most systems, this means using an x86-64 (aka AMD64) installation; however, some older Intel-based Macs use 32-bit EFIs.

Should You Put Off Trying GPT?

If you're considering an MBR-to-GPT conversion, or even using GPT on a "fresh" disk that's never been partitioned, the fact that there can be problems such as I've described may give you pause. You may want to proceed with using GPT only if you have a ready means of recovery, such as access to a USB adapter so that you can plug the disk into the computer after it's booted another OS or a recovery CD. It should then be possible to recover the disk by making small changes to the partition table or boot loader, as described above. Keep in mind that, although I've encountered these problems once or twice, I own or owned six BIOS-based computers that use one or more GPT disks, and I've booted most of these with at least two or three different GPT layouts, so the incidence of problems seems to be no more than about 10%, and probably less than that. GPT-partitioned disks have been used for years on BIOS-based computers, particularly in some communities, such as the Hackintosh (Mac OS X on commodity PC hardware) community. Reports of GPT/BIOS incompatibilities in such communities are rare, suggesting that even a 10% problem incidence is a gross overestimate. On the other hand, it's possible that users are misattributing problems to other causes, such as boot loader bugs. Also, hybrid MBRs are common on Hackintoshes, which could help keep BIOS incompatibility problems in check.

If your computer is new, chances are it uses an EFI rather than a BIOS, even if it boots in BIOS/CSM/legacy mode. On such computers, my preference is to boot in EFI mode using GPT; however, Linux's EFI support is still relatively new, and many EFIs are buggy, so a BIOS/CSM/legacy-mode boot does have its advantages, particularly if you're uncomfortable with EFI-mode booting. Such computers are rather likely to have problems with BIOS/CSM/legacy-mode booting with GPT, although in most cases adding the MBR boot flag or deleting the ESP will work around the problem.

Overall, I recommend you keep the potential for problems in mind, and have a recovery plan ready, but don't abandon plans to move to GPT if such a move presents real benefits to you.

If you've encountered this problem and have additional information to contribute concerning its causes or how to fix it, I'd like to hear from you. You can e-mail me at rodsmith@rodsbooks.com.

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