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#1 2013-02-14 23:21:06

Pyrobisqit
Member
Registered: 2011-05-26
Posts: 23

GPT on BIOS through MBR on a secondary HDD

So I really want to try out the new GPT partitioning scheme, but I'm stuck with a BIOS, and will be for at least 2 or 3 more years before I can afford a new computer.

So the thing is... I've been thinking of easy and elegant solutions to solve this GPT BIOS booting problem, and I came to a conclusion:

Since BIOSes can boot MBR-formatted drives, what if I were to use my /dev/sdb with MBR to put GRUB there, and then make GRUB load my GPT-formatted HDD on /dev/sda, would that even be possible?

Also, /dev/sdb will (soon, I'm getting it ready) a Windows partition.

Since both drives are right now empty, is there a way to install Arch with everything onto /dev/sda EXCEPT the bootloader and MBR stuff, and just throw GPT in, and then install GRUB on /dev/sdb? This way I'll also be able to dual boot Windows and Linux. Is there a way to do this elegantly, preferably all from the Arch iso I downloaded onto my pendrive? Like, can I install GRUB with the standard ISO Arch provides without much tweaking? Can you guys show me a guide on how to install GRUB to a different HDD than the one containing the root partition?

Thanks much!

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#2 2013-02-15 01:53:01

sano
Member
Registered: 2012-02-11
Posts: 113

Re: GPT on BIOS through MBR on a secondary HDD

Maybe this helps? I am not sure if I really understand what you are trying to do...

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#3 2013-02-15 11:57:45

Pyrobisqit
Member
Registered: 2011-05-26
Posts: 23

Re: GPT on BIOS through MBR on a secondary HDD

sano wrote:

Maybe this helps? I am not sure if I really understand what you are trying to do...

I'll try to clarify...

/dev/sda contains /dev/sda1 (root) and /dev/sda2 (home), it's GPT.
/dev/sdb is empty, MBR.

Since my BIOS doesn't support GPT, the idea would be to install GRUB-legacy/GRUB2/Syslinux (whatever works) to /dev/sdb's MBR, and boot Arch Linux contained in /dev/sda1, without installing any bootloader in /dev/sda preferably. Is this kind of hybrid MBR/GPT setup possible? And more importantly, will my BIOS read GPT-formatted drives larger than 2'2 TBs?

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#4 2013-02-15 12:04:34

MFserver
Member
From: Sweden
Registered: 2011-10-28
Posts: 43
Website

Re: GPT on BIOS through MBR on a secondary HDD

I'd say it is probably not possible, but I am not completely sure. If the IO to the disk goes through BIOS even after boot, it is not possible. You should be able to test it by formatting a drive as GPT, and then trying to access it from a running Linux installation on the machine. If that works, you have a chance of getting things to work smile


Sometimes, when I'm trying to get any audio software or hardware working with my system, I wonder why I ditched Windows. But every time I work at a windows computer, I remember it again.

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#5 2013-02-15 12:15:52

Foucault
Member
From: Athens, Greece
Registered: 2010-04-06
Posts: 209

Re: GPT on BIOS through MBR on a secondary HDD

I am not exactly sure I understand your situation but in my system I have archlinux on a GPT-partitioned disk booted by syslinux and a second MBR-partitioned disk that contains windows. So, you can put the bootloader in the GPT disk and when/if you install any other OS on the MBR-one you can have syslinux (or grub2) to chainload into it.

Other than that, linux can boot from a GPT disk even on BIOS systems (as long the bootloader supports it).

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#6 2013-02-15 14:02:51

Pyrobisqit
Member
Registered: 2011-05-26
Posts: 23

Re: GPT on BIOS through MBR on a secondary HDD

Foucault wrote:

Other than that, linux can boot from a GPT disk even on BIOS systems (as long the bootloader supports it).

Apparently not, as stated by the Arch Wiki on the Syslinux and GRUB2 guides. My machine would just halt where it's supposed to be booting that GPT disk. It won't even throw errors, it will just freeze and do nothing, not even a beep. While, on MBR, GRUB2 and Syslinux have always worked fine, which leads me to believe it's definitely my motherboard and not my HDD or my bootloader.

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#7 2013-02-15 15:35:01

the.ridikulus.rat
Member
From: Indiana, USA
Registered: 2011-10-04
Posts: 765

Re: GPT on BIOS through MBR on a secondary HDD

Pyrobisqit wrote:
sano wrote:

Maybe this helps? I am not sure if I really understand what you are trying to do...

I'll try to clarify...

/dev/sda contains /dev/sda1 (root) and /dev/sda2 (home), it's GPT.
/dev/sdb is empty, MBR.

Since my BIOS doesn't support GPT, the idea would be to install GRUB-legacy/GRUB2/Syslinux (whatever works) to /dev/sdb's MBR, and boot Arch Linux contained in /dev/sda1, without installing any bootloader in /dev/sda preferably. Is this kind of hybrid MBR/GPT setup possible? And more importantly, will my BIOS read GPT-formatted drives larger than 2'2 TBs?

Yes, this kind of setup is possible with GRUB2. You should use UUID or PARTUUID in grub.cfg though to make sure sda vs sdb issues don't crop up. I don't know whether Syslinux or GRUB-Legacy support such cases. But first I would like to know what exactly is the issue in installing the bootloader to /dev/sda itself? What errors are you getting in booting /dev/sda in BIOS-GPT mode without the help of /dev/sdb?

Last edited by the.ridikulus.rat (2013-02-15 15:35:29)

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#8 2013-02-15 17:19:06

Foucault
Member
From: Athens, Greece
Registered: 2010-04-06
Posts: 209

Re: GPT on BIOS through MBR on a secondary HDD

Pyrobisqit wrote:

Apparently not, as stated by the Arch Wiki on the Syslinux and GRUB2 guides. My machine would just halt where it's supposed to be booting that GPT disk.

Well it seems you might be unlucky enough to have a quirky BIOS. Personally I have two systems with GPT/BIOS and they boot fine with syslinux. Although I believe it is too much trouble to do what you are thinking just to install Arch in GPT your proposed solution should work.

Also, as far as I understand, BIOS does not "read" the disks. This is up to the OS. If you can see the disk when booting from the live usb then you are good to go.

Last edited by Foucault (2013-02-15 17:19:43)

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#9 2013-02-15 19:11:27

srs5694
Member
From: Woonsocket, RI
Registered: 2012-11-06
Posts: 719
Website

Re: GPT on BIOS through MBR on a secondary HDD

Pyrobisqit wrote:
Foucault wrote:

Other than that, linux can boot from a GPT disk even on BIOS systems (as long the bootloader supports it).

Apparently not, as stated by the Arch Wiki on the Syslinux and GRUB2 guides.

If a wiki says that BIOS-based computers can't boot from a GPT disk, then the wiki is wrong. I've done it on several computers in the past, although I'm now using native (U)EFI on most of my systems. That said....

My machine would just halt where it's supposed to be booting that GPT disk. It won't even throw errors, it will just freeze and do nothing, not even a beep. While, on MBR, GRUB2 and Syslinux have always worked fine, which leads me to believe it's definitely my motherboard and not my HDD or my bootloader.

There are buggy BIOS implementations that have problems with GPT. Note that this is definitely a BIOS bug; the BIOS, contrary to popular opinion, is not required to interpret partitions in any way. Some do so, either because they're actually EFIs in BIOS emulation mode or because they have enough "smarts" to read partition tables so as to be able to read firmware updates from USB flash drives or the like. Most such BIOSes still have no problem booting from a GPT disk, but a few do have problems. These problems can usually be worked around by setting the boot flag on the 0xEE protective partition in the protective MBR or by some other means. See my gdisk documentation on BIOS for details.

Note also what I mentioned earlier: Many of the afflicted BIOSes seem actually to be EFIs. They sometimes default to booting in BIOS mode, but they do contain EFI boot capabilities. Intel, for instance, has been selling EFI-based motherboards for years, although they haven't been making a big deal of the EFI capabilities. Thus, you might want to study your firmware options and/or your manual for mention of EFI. (Searching a PDF manual can be a handy way to do this.)

Another point is that Windows can't boot in BIOS mode from a GPT disk. I suspect you're aware of this, but I don't want anybody to walk away from this thinking that a Windows installation can be converted to GPT form and still boot in BIOS mode. (It's possible to do such a conversion in conjunction with DUET, which is essentially a software-based EFI, but this is awkward to set up and doesn't work on every computer.)

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#10 2013-02-15 19:30:02

Antoine
Member
From: Picton Ontario
Registered: 2012-10-11
Posts: 90

Re: GPT on BIOS through MBR on a secondary HDD

Foucault wrote:

I am not exactly sure I understand your situation but in my system I have archlinux on a GPT-partitioned disk booted by syslinux and a second MBR-partitioned disk that contains windows. So, you can put the bootloader in the GPT disk and when/if you install any other OS on the MBR-one you can have syslinux (or grub2) to chainload into it.

Other than that, linux can boot from a GPT disk even on BIOS systems (as long the bootloader supports it).

I do it with Grub2, on an older Core 2 Duo laptop.

I have a Dell D630, 2 HDDs. /dev/sda is protective MBR, GPT. sda1 is the mini-boot partition that Grub2 wants; sda2 is swap; sda3 is / in ext4 with 64-bit Arch (I don't wish to discuss my partitioning scheme; I would't have added the swap if I'd had 2 more gigs RAM). /dev/sdb is NTFS XP.

Any rate, following the trusty Beginners Guide, I installed Grub2 with os-prober and it's very clever-boots: it automatically brought up the boot screen with an option to boot from /dev/sdb if I have some strange need for that (the laptop came "refurbished" from Dell with an XP Pro installer disc, so I used it while I was buying a drive and caddy for Arch). It works, too; I tried it. Of course, the clock comes up UTC in Windows ... (I know how to fix it, I just haven't yet). I've never ever used any form of Windows as a day-to-day OS.

Now, when I tried to install a hackintosh system (one that's known to work with this machine) the thing just froze with two lines of "boot error" (something about GPT) so I gave up on that and went with Arch. The best way after all. The drivers all "just work" for one thing.

Just to say, it can be done; this is an older BIOS machine.

Edit: added link to grub-bios section of Beginners Guide.

Last edited by Antoine (2013-02-15 19:40:52)

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