Dual boot EFI and Legacy BIOS

OK, the threads here typically discuss dual booting from separate partitions or discs. But nearly all do so in the context of a single platform, be it EFI or the classic BIOS. For various reasons I'd like to know the risks and chances for success of doing this across both systems.

Meaning: I've an OEM Win 7 Home pre-installed on an EFI/gpt disc. On the other internal mbr disc I'd like to place Win 7 Pro using the Legacy BIOS. Yes, I know I could convert the mbr to gpt and let EFI run the whole show, but then this would not be a 'true' and independent dual boot would it? So, will this work? Will there be conflicts? Will the gpt disc see the mrbr disc and vice versa? Will the respective OSs NOT overwrite the other's boot files/partitions and remap memory? In a word, will a state of democracy prevail on my computer?

I'm willing to forego the EasyBCD route in regards to the dual boot screen and simply use Boot override esc key on my Asus when I want to boot into the MBR disc. Thanks for your help.

 

Flipp, that's pretty much the consensus I've gotten everywhere: nobody really knows. No one's tested it. My Aptio designed BIOS/motherboard also gives me the choice on a one-time boot as to whether I want run my USB drive on through the default UEFI Win 7 boot loader or Legacy. But that misses the whole point. It assumes that the dual installation has already worked. It doesn't answer the base question as to whether even if the motherboard has allowed you to fork over, once, into your second disk under, say, Legacy/mbr, once you're there will that disk read anything on the other gpt disk and/or will it already have erased the boot partition on that first EFI disk. I mean it's one of those either very obvious or very obscure questions. But, unfortunately I've yet to find a single post over three well-respected tech sites such as yours that has been able to give a cut and dry answer about it. EFI + EFI or BIOS + BIOS = little or no problem. Yet Flipp, with all respect, your answer typifies the confusion about this issue in a nutshell: EFI + BIOS = er, well, I'm not sure. I could become my own guinea pig, of of course, with all the risks, but it seems hard to imagine with all those guru hackers out there that this problem hasn't already been debugged long ago.

 

Flipp and tnx, thanks to you both, I finally found people who are at least tuned into the problem. It partly boils down to what uefi/legacy hybrid bios means and how to recognize it in your bios. A post on another site has given me some intuition into the matter. My Asus N76vm has the American Megatrends v211 which doesn't have an option to 'Launch UEFI'. It does have a configuration mode 'Launch PXE OpRom' with an info window that explains 'Controls the execution of UEFI and Legacy PXE OpRom'. Does PXE OpRom = Legacy BIOS, or is this something else. Oddly, an earlier version of the BIOS v207 carried an instruction 'Launch UEFI' in place of PXE Oprom line. The new BIOS v214 (which I haven't dared to touch yet) carries the following description on Asus site; "Disable CSM when Secure Boot is enabled". I've learned that the 'Launch CSM (Compatibility Support Mode)' is inserted in place of the former 'Launch UEFI' instruction. Does this imply that the three seemingly different options under the Boot Configuration tab are basically saying the same thing: you can choose between pure UEFI and a hybrid mode that is compatible with the classic BIOS? And what's the business about secure boot on a Windows 7 machine that isn't supposed to have it? I texted Asus support about this two days ago and have yet to hear anything from them.

 

Following my preceding post I'd like to walk you through what I think may result from three different lines of action:

A: From Windows 7 disc 0 (with both discs connected) convert my disc 1 to gpt and install 2nd version of Win 7 (or Win 8 or even Vista). PROS: easy, fairly fail safe, Windows Boot Loader will automatically take care of dual boot screen configuration under UEFI. CONS: This isn't a 'true' dual boot, so when disc 0 dies the Windows install on disc 1 dies with it.

B: With disc 0 disconnected install Win 7 (or 8 or Vista) on disc 1 using the pre-formatted mbr partitions and with boot info on the same disk. PROS: True dual boot, disk 0 dies and I'm up and running on disk 1 immediately. CONS: No user-friendly Grub, obligation to boot from BIOS override, rearranged partition letters (minor cosmetic nuisance). Also, risk that disc 1 doesn't see disc 0 or worse that Windows Uefi loader will somehow expropriate the boot info from the mbr disk and place it in its own EFI partition (unknown but probably doubtful). As further input, I tried to make an Acronis 11 backup (made on XP3 comp but supposedly UEFI-capable) and although disc booted successfully could not see disc 0 at all. But this might be an Acronis problem.

C: Convert disc 1 to gpt AND disconnect disc 0 before installing Win 7 ( or 8 or Vista) onto disc 1 in UEFI mode. This should result in an EFI partition on each HDD. PROS: A coherent ideal solution with the best of both worlds. CONS: Potentially the worst of both worlds. As the EFI mode-for-all now allows everyone to see what everyone else is doing on the computer, a serious risk that EFI O will hijack EFI 1 boot, vice versa, or even that one or both installations will simply implode.

I welcome your educated guesses regarding these scenarios or any better plan of attack.

Also tnx, yeah, I'd be simply thrilled to hear a detailed solution if you manage to get XP3 up and running alongside Win 7 on a separate MBR disc. It's probably the most difficult combo of all. I was told flat out by my vendor that my Asus machine simply doesn't support it. But from some research I've found that apart from USB 3 drivers just about everything else should run albeit perhaps on crutches.

Yes, but are those strictly data discs? I've the same. The real question is what if you hotswap a non GPT disk with another OS on it?

 

Ah really great tnx. Appreciate your effort in helping get to the bottom of this. So in fact you've tested my B) option and it apparently seems to work. What's not clear from your test is...can either disk perfectly read the others partitions? Naturally, each Win 7 version boots as C: off its respective disk but what about the rest. Also when both disks are installed, have you tried just letting the Windows Boot Loader do its thing without going into BIOS first? What happens then? As for XP, yeah, it's the classic SATA driver issue which I think one fixes by either a slipstream or pressing F6 at one point in the install and pointing the process to a USB key with the drivers loaded.

I called Asus support three times, twice here in France, once in the U.S. Of the former, first I got a North African lady who sounded as she was coming in off a cocktail party who advised nothing more than: "Backup your data". LOL. The second French intervention came from a trilingual miss of Asian origin who told me flat out that she'd try and test the solutions at home when she gets off work. Nearly fell off my chair. And one wonders that China has put a man in space and has built the world's longest high speed train? In the U.S. I was rerouted to somewhere in Pakistan (amazing how far a dime will get you on a phone call these days). Anyway, after a half dozen holds to 'consult with my supervisor' (who was probably in Indiana) I was told simply in a robotic monotone: "Sir, Asus does not support dual booting its machines, we value our customers, have a nice day, etc." Isn't culture such a marvelous thing?

Anyway, tnx I think we're nearly at the bottom of all this. And I'll have a go on my single machine this weekend.

 

Brilliant, tnx, just brilliant. I'll have you know that to my mind this specific setup hasn't previously been verified by anyone on any of the forums I've been connected to over the past couple of weeks. There remains the niggling question as to why when you installed via UEFI first the Legacy install wouldn't take. This is a bit of a bugger. Also your experience seems to imply that if the BIOS is set to boot off the MBR disc first then EasyBCD will allow either disk to boot whereas if the GPT disk is set to boot first (BIOS-wise) then EasyBCD will only let the UEFI disk boot. I wonder if this depends on what's going on within EasyBCD. Didn't I read somewhere that EasyBCD should only be on one disk, notably the last one installed? You also say you copied the exe file from one to the other. Is this same as installing on the other disk? To me the fundamental question is where is the information stored that EasyBCD uses. I ask this having never even used it myself. It must be on one of the disks, obviously, and depending upon where the dual boot info is, will execute things or not. I even wonder if somewhere along the line boot files have been overwritten somewhere.

Can you still 'manually' boot either disk off the BIOS? One interesting test would be to remove the x32 disk and see if the 64 bit EFI install will still boot automatically. Also, when you set the x64 EFI 'to boot first in the BIOS' what was being prioritized: disk 0, disk 1 OR Windows Boot Loader? I'd be curious to know whether if Windows Boot Loader is allowed to kick in off the 64bit install that this would make a big mess of things. But again I'm just babbling in the woods, again. I've got enough encouragement now to do my own test, this time with two versions of x64. Anywhere, thanks so much for your efforts in getting into this new territory. And I'll be sure myself to report back on my own experience.

 

Absolutely agree, tnx, when I found the way the one-time boot function works (I, too, have an Asus) I find it just as simple to use as the EasyBCD interface. It's clearly more accurate to boot (no pun intended). Easy BCD can be prone to false-positive errors especially if the user configures it wrongly. I've seen this problem with others on various web sites. Sticking with the BIOS is straight and to the point, and you have one less intermediary where things can go afoul.