[SOLVED] UEFI dual boot on separate HDD'S

haVok1701 · 2013-05-10 13:47:04

I'll have arch and windows 7 on separate hdd's and using reEFIed for booting arch, so the question is should I only have one ESP (on the arch disk) or should the window's disk also have it's own ESP?

Last edited by haVok1701 (2013-05-10 18:18:44)

srs5694 · 2013-05-10 16:27:44

According to the EFI specs, either way is fine. Windows has problems with multiple ESPs on a single disk; the installer gets confused, and the OS might get confused in some operations, too, although I'm not positive of that. I don't know how Windows responds with multiple disks, each of which has its own ESP. Thus, if you don't want to experiment to discover the answer to this question, I'd just go with a single ESP for both OSes. If you try it the other way and run into problems, you should be able to work around them by temporarily changing the type code of the ESP you don't want Windows to use to something else. (Using "8300" in gdisk will keep it out of Windows' view and therefore safe from tampering.)

the.ridikulus.rat · 2013-05-10 17:03:08

According to UEFI spec., each disk (SSD or HDD) should have atleast one ESP. The design of ESP allows for different OSes to store their boot files without overwriting one-another's files. So, although the UEFI spec. allows for multiple ESPs in a single disk, the preference is for only one ESP per disk.

haVok1701 · 2013-05-10 18:11:44

Ok thanks

srs5694 · 2013-05-10 23:02:06

the.ridikulus.rat wrote:

According to UEFI spec., each disk (SSD or HDD) should have atleast one ESP.

Can you point to where the spec says this? I couldn't find such a claim in a quick search. I did find this, though, which seems to contradict what you say:

EFI_2_3_1_spec wrote:

UEFI does not impose a restriction on the number or location of System Partitions that can exist on a system.
...
Software installation may choose to create and locate an ESP on each target OS boot disk, or may choose to create a single ESP independent of the location of OS boot disks and OS partitions.

That's from the 2.3.1 EFI spec, p. 492 (section 12.3.3). The second sentence, in particular, strongly suggests that a computer with two disks can have either one ESP between them or one ESP per disk (hence two ESPs total); either approach seems to be allowed. The first sentence, of course, suggests that a single disk can host multiple ESPs -- but as I said, Windows reacts badly to this type of configuration, so I don't recommend it, even if it's valid by the EFI spec.

the.ridikulus.rat · 2013-05-11 07:58:07

srs5694 wrote:

the.ridikulus.rat wrote:
According to UEFI spec., each disk (SSD or HDD) should have atleast one ESP.
Can you point to where the spec says this? I couldn't find such a claim in a quick search. I did find this, though, which seems to contradict what you say:

I remember reading this somewhere in 2.0 or 2.1 version of spec. AFAIK OS installers expect (and try to install to) an ESP to be present in the same disk as the OS is installed in, even if any other disk has a valid ESP, i.e. the same way they try to install the boot code to the same disk's MBR (by default) in case of BIOS boot. Things might have changed now. In any case its better to have at least one ESP per disk.

Last edited by the.ridikulus.rat (2013-05-11 08:03:35)

the.ridikulus.rat · 2013-05-11 08:09:42

srs5694 wrote:

EFI_2_3_1_spec wrote:
UEFI does not impose a restriction on the number or location of System Partitions that can exist on a system.
...
Software installation may choose to create and locate an ESP on each target OS boot disk, or may choose to create a single ESP independent of the location of OS boot disks and OS partitions.
That's from the 2.3.1 EFI spec, p. 492 (section 12.3.3). The second sentence, in particular, strongly suggests that a computer with two disks can have either one ESP between them or one ESP per disk (hence two ESPs total); either approach seems to be allowed. The first sentence, of course, suggests that a single disk can host multiple ESPs -- but as I said, Windows reacts badly to this type of configuration, so I don't recommend it, even if it's valid by the EFI spec.

http://msdn.microsoft.com/en-us/library … 63525.aspx

Microsoft's Windows and GPT FAQ wrote:

Q. Can there be two ESPs on a single disk?
A. Such a configuration should not be created and is not supported in Windows.
Q. What about two ESPs on two different disks?
A. ESP partitions can be replicated for high-availability configurations. Replication must be done manually and the contents must be synchronized manually when using software volumes. Hardware vendors may provide additional solutions for high availability. ESP partitions cannot be mirrored.
Q. Where should the ESP be placed on the disk?
A. The ESP should be first on the disk. While there is no architectural requirement, there are numerous reasons why it is beneficial to place the ESP first. The primary reason for this is that it is impossible to span volumes when the ESP is logically between the two data partitions that you are attempting to span.

The 3rd question relates to booting Windows via Dynamic disks (Windows equivalent of LVM/RAID). If the user does not require the disk to be Dynamic type, then the ESP can be located anywhere in the disk. Both Linux and Windows have no issues with ESP not being 1st partition in the disk (I have tested such dual-boot config without issues).

mrunion · 2013-05-11 15:08:01

FWIW, I have two ESPs on a single disk and Windows 8 boots just fine.

(Microsoft needs to stop playing by their own rules all the time, and get with the "program".)

srs5694 · 2013-05-11 15:50:22

the.ridikulus.rat wrote:

I remember reading this somewhere in 2.0 or 2.1 version of spec

I just searched a copy of the 2.0 specification and couldn't find a claim that each disk (or even each bootable disk) must contain an ESP.

Microsoft's Windows and GPT FAQ wrote:
Q. Can there be two ESPs on a single disk?
A. Such a configuration should not be created and is not supported in Windows.

This is Microsoft being more restrictive than what the EFI spec explicitly permits, as I've already stated.

Q. What about two ESPs on two different disks?
A. ESP partitions can be replicated for high-availability configurations. Replication must be done manually and the contents must be synchronized manually when using software volumes. Hardware vendors may provide additional solutions for high availability. ESP partitions cannot be mirrored.

Note the use of the term "can be replicated" -- not "must be replicated." In other words, Microsoft permits each disk to have its own ESP, but does not require it to do so. Of course, what the Microsoft document says and what the software actually does may be different. Furthermore, what Microsoft says and what the EFI spec say don't always sync up....

Q. Where should the ESP be placed on the disk?
A. The ESP should be first on the disk. While there is no architectural requirement, there are numerous reasons why it is beneficial to place the ESP first. The primary reason for this is that it is impossible to span volumes when the ESP is logically between the two data partitions that you are attempting to span.

By "no architectural requirement," Microsoft means that they're laying out a more stringent requirement than what's in the EFI spec. Note the quote from the actual EFI spec that I produced earlier -- "UEFI does not impose a restriction on the number or location of System Partitions" (emphasis added).

It should also be noted that many Windows 8 computers are shipping on which the ESP is not the first partition on the disk, thus violating Microsoft's own recommendation on this score. On such systems, the manufacturer typically puts one or two manufacturer-specific partitions before the ESP. Sometimes, one of these partitions partially replicates the functionality of the ESP, holding boot loader files. This can complicate OS installation and setup, and it makes it harder for people like you and me to offer support to the people who have such systems, since it's not always obvious what all the partitions and files do.

mrunion wrote:

FWIW, I have two ESPs on a single disk and Windows 8 boots just fine.

Did you install Windows 8 with two ESPs, or create the second ESP after installing Windows? Installation is where I've run into problems with Windows 7 -- the installer becomes confused when a disk holds two ESPs. I haven't tested, but there may be problems with mounting the ESP after installation, too. In Windows, the "mountvol" command-line command has a flag ("/S") to mount the ESP. It can be used like this:

mountvol S: /S

This example mounts the ESP to S:. I have no idea what would happen if you'd use this command on a system with two ESPs (on the same or different disks).

mrunion · 2013-05-11 22:52:38

@srs5694. Indeed, it was AFTER Windows 8 was already present.

Arch Linux

#1 2013-05-10 13:47:04

[SOLVED] UEFI dual boot on separate HDD'S

#2 2013-05-10 16:27:44

Re: [SOLVED] UEFI dual boot on separate HDD'S

#3 2013-05-10 17:03:08

Re: [SOLVED] UEFI dual boot on separate HDD'S

#4 2013-05-10 18:11:44

Re: [SOLVED] UEFI dual boot on separate HDD'S

#5 2013-05-10 23:02:06

Re: [SOLVED] UEFI dual boot on separate HDD'S

#6 2013-05-11 07:58:07

Re: [SOLVED] UEFI dual boot on separate HDD'S

#7 2013-05-11 08:09:42

Re: [SOLVED] UEFI dual boot on separate HDD'S

#8 2013-05-11 15:08:01

Re: [SOLVED] UEFI dual boot on separate HDD'S

#9 2013-05-11 15:50:22

Re: [SOLVED] UEFI dual boot on separate HDD'S

#10 2013-05-11 22:52:38

Re: [SOLVED] UEFI dual boot on separate HDD'S

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