"Windows" and "EFI" boot options question

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Level 1

13 points

"Windows" and "EFI" boot options question

Hello,

I'm using two 5k retina iMacs, a late-2014 and a mid-2017 model.

Occasionally, I run Windows on both of them from an external Thunderbolt SSD (I hate to have windows stuff on my internal drives so I run it from the external drive).

Both iMac can run the same Windows installation from that external drive without issues.

But the boot screens look different on the machines. The 2014 iMac shows "Windows" and "EFI" when I boot. The "Windows" option boots Windows normally. If I select the "EFI" option, it shows me a blue screen saying:

Recovery
Your PC needs to be repaired
A required device isn't connected or can't be accessed.

Press Enter to try again
Press F8 for Startup settings

When I tried pressing Enter and F8 keys, it returned to the same blue screen.

On the 2017 machine, I can see an "EFI" option only and it boots Windows normally.

I'm certain that none of the machines have any Windows stuff on it.

Why do the boot screen look different on the iMacs even though I use the same external SSD drive with both of them? Can I do something to make it the same on both?

late-2014 5k retina iMac:

mid-2017 5k retina iMac:

Thank you

iMac with Retina 5K display, null

Posted on Nov 8, 2017 2:49 PM

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Answer 1

Top-ranking reply

Loner T

Level 9

59,513 points

Nov 23, 2017 3:02 PM in response to DiamondLINEx

The 2014 iMac is a UEFI Mac which also supports CSM-BIOS, so it can support either BIOS or EFI. On a 2017 Mac, the CSM-BIOS layer is no longer available. The Hybrid MBR is a vestigial construct on the SSD on a 2017 Mac.

Using BCDboot Command-Line Options (see the explanation under the "/s" qualifier) and BCDEdit Command-Line Options , you can see where the BCD points to for each model - BIOS or EFI.

You seem to have a BCD for each type on your SSD, and with the Hybrid MBR in place, it works correctly on both Mac models.

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Answer 2

Loner T

Level 9

59,513 points

Nov 8, 2017 3:19 PM in response to DiamondLINEx

You are breaking quite a few rules.

2014 iMac supports EFI and legacy BIOS, 2017 iMac does not. Post the output of sudo fdisk /dev/diskN where 'N' points to the external SSD. You can find 'N' by looking at the output of diskutil list.
Your Windows drivers are hardware specific. The 2014 and 2017 iMacs have different GPUs, even if the same drivers 'work'.
Portable installation of Windows is part of Windows-to-Go under Windows Enterprise. WE is not supported on Macs. Your retail 'license' is fairly clear about this.
If you installed W10 on the 2017 first, there is an EFI partition on your internal disk with a Microsoft directory.
You may end up with a corrupt BCD sooner than later, which will cause new issues. I suggest you backup Windows on a regular basis.

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Answer 3

DiamondLINEx Author

Level 1

13 points

Nov 8, 2017 3:40 PM in response to Loner T

1.

Disk: /dev/disk3 geometry: 30401/255/63 [488397168 sectors]

Signature: 0xAA55

Starting Ending

#: id cyl hd sec - cyl hd sec [ start - size]

------------------------------------------------------------------------

*1: 07 0 32 33 - 1023 254 63 [ 2048 - 488393010] HPFS/QNX/AUX

2: EF 1023 254 63 - 1023 254 63 [ 488395058 - 512] <Unknown ID>

3: 00 0 0 0 - 0 0 0 [ 0 - 0] unused

4: 00 0 0 0 - 0 0 0 [ 0 - 0] unused

2. Well, at first windows did not recognize the mid-2017 iMac GPU but I reinstalled the BootCamp Assistant and now it recognizes the Radeon Pro GPU. When I switch back to the previous iMac, I do not experience any issues but I will no longer use the 2014 iMac with it to avoid any issues.

4. I never installed Windows on the 2017 iMac. I created the external SSD windows installation using the 2014 iMac. Then I rebuilt its fusion drive following your guide. I use this particular Windows installation since then with both machines.

5. I only use my external SSD Windows installation for gaming, I do not store important information on it but I will stop using the 2014 iMac with it to avoid any issues.

Thank you!

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Answer 4

Loner T

Level 9

59,513 points

Nov 8, 2017 3:51 PM in response to DiamondLINEx

You have an MBR formatted disk. The Boot directory is contained on the external disk (C:\Boot). Step 2 explains how it works on both machines.

On the 2014 iMac, it expects the EFI selection to go to the ESP (Mac's EFI partition) and does not have anything which has the Boot Loader and Boot Manager.

Based on what you have posted so far, the legacy Windows boot on 2014 is more suitable for the external SSD due to it's MBR. Also, can you run msinfo32 on each machine when booted in Windows and check BIOS Mode value?

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Answer 5

DiamondLINEx Author

Level 1

13 points

Nov 17, 2017 6:57 AM in response to Loner T

BIOS Mode: UEFI

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Answer 6

Loner T

Level 9

59,513 points

Nov 17, 2017 8:34 AM in response to DiamondLINEx

DiamondLINEx wrote:

BIOS Mode: UEFI

Is this on the 2014 Mac or the 2017 Mac?

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Answer 7

DiamondLINEx Author

Level 1

13 points

Nov 17, 2017 8:55 AM in response to Loner T

it's on the 2017 iMac. I did not use this installation with the 2014 iMac since your recommendation not to use it with multiple machines.

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Answer 8

Loner T

Level 9

59,513 points

Nov 17, 2017 9:02 AM in response to DiamondLINEx

As long as the SSD works properly on the 2017 Mac, without any driver issues, continue to use it. You will not see the Windows icon on 2017 Mac due to lack of BIOS supports in the firmware.

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Answer 9

DiamondLINEx Author

Level 1

13 points

Nov 23, 2017 9:32 AM in response to Loner T

so you say it's perfectly normal that I see different boot options on the two machines and that it is due to different BIOS support?

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