How to boot OpenCore on a 2010 iMac with macOS Sierra

[amazzi44]

Hi all,
please consider me as a dumb,
I followed all the steps from this forum to upgrade the GPU of my 21.5" mid-2010 iMac running Sierra to a Nvidia K1100M.
Everything went well during the hardware and vBios flashing procedure, but now I can't slect the booting disk and I have lost brightness control.
I know that the solution is provided by OpenCore, but I can't still understend how it works. I've tried to make a bootable USB stick with Catalina Loader and it didn't boot; I did the same by creating another partition with Catalina Loader as well but it didn't help.
I looked around and I saw OpenCore Legacy Patcher, but I'm still very confused, since I don't plant to upgrade my macOS unless it would be necessary.
Any kind of help would be insanely helpful, since I'm stuck.
Thank you very much in advance.

 

[Nguyen Duc Hieu]

If you are bold enough, just overwrite the EFI folder generated by OCLP on the hidden EFI volume on your High Sierra disk (I did this).

The OCLP configurator (text base user interface) has this function from its menu.

If you're afraid of corrupting your HS volume do as below:
1. Format a USB stick and make it bootable ("bless") on your iMac. Your should see it as sellectable boot device on the desktop and System preference => Start up disk.
2. Generate EFI folder by OCLP and copy it to the USB stick.
3. Open System preference and switch the boot disk to the newly patched USB stick.

 

[amazzi44]

Thank you indeed for your reply! So if I corrupt my EFI volume on my disk won’t I be able to restore it by a fresh install via Command+R and so on?
I would prefer this solution but I’m afraid I would do a mess.
About the second solution, will it work if the USB stick has nothing else on but the EFI partition?
Nevertheless every time I tried to select the USB stick as startup disk the System Preferences told me:

You can’t change the startup disk to the selected disk. The bless tool was unable to set the current boot disk.

SIP is disabled.
I have FileVault enabled and an “old” Windows 7 partition that I could remove if necessary.
Maybe a clean install would help?

 

[Nguyen Duc Hieu]

Thank you indeed for your reply! So if I corrupt my EFI volume on my disk won’t I be able to restore it by a fresh install via Command+R and so on?
I would prefer this solution but I’m afraid I would do a mess.
About the second solution, will it work if the USB stick has nothing else on but the EFI partition?
Nevertheless every time I tried to select the USB stick as startup disk the System Preferences told me:

SIP is disabled.
I have FileVault enabled and an “old” Windows 7 partition that I could remove if necessary.
Maybe a clean install would help?

Sorry, I don't have the practice to restore anything on my disk so I can't answer your question.
I have a bad habit of totally wipe out my disk and freshly re-install OS from a USB installer.
Another option is to store a fresh installed disk somewhere, when in need, I just carbon copy it back to the internal SSD, faster than re-install OS.
And by all means, what do you need Filevault for?

 

[amazzi44]

FileVault - let’s say - just in case something happens. Yeah, maybe the best thing is to disable it during these procedures…
Tomorrow I’m going to try the brave option: do you know if I just need to select my drive as target in the OCLP text based menu, and then pray?

 

[Nguyen Duc Hieu]

1. Remember to back-up your data before proceeding with anything.
2. If you have a USB stick/SD card around, try it first. With the K1100m, you can hold Option at boot to select booting from the USB stick/SD card to test its function (brightness control etc.)

 

[amazzi44]

Thanks for your help! Updates will follow.

 

[amazzi44]

Some updates.

https://imgur.com/a/abMTpEw

This is what I did for making my OC USB Stick.
Then

https://imgur.com/a/5RVqydC

I managed to get OC USB stick to boot by pressing Alt, but when I try to enter my sistem it gives me a prohibted sign.
When I boot my regular Machintosh HD it goes normally without brightness control.

 

[Nguyen Duc Hieu]

It's strange to me. Maybe you need some old version of OCLP rather than the latest one.
OCLP version 0.2.2 worked well for my iMac 2009 (K1100m) High Sierra, so I didn't use newer version.
Remember to use the Text base (TUI) version instead.
You need to do some fine tune on the menu before generating the EFI and copy it to the USB.

Below are the two important link to OCLP

1. Guide

OpenCore Legacy Patcher

Experience macOS just like before

dortania.github.io

2. Download link

Releases · dortania/OpenCore-Legacy-Patcher

Experience macOS just like before. Contribute to dortania/OpenCore-Legacy-Patcher development by creating an account on GitHub.

github.com

 

[amazzi44]

I succeeded in getting it working by installing OCLP directly to my internal drive.
Then I tried to upgrade to Catalina but I had issues with the graphics (very laggy and definitely unusable, picture attached).
In the end I needed to make a High Sierra install drive from another Mac, and after installing now it’s working properly.
Such a pity Catalina didn’t work: any ideas?

 

[Nguyen Duc Hieu]

I succeeded in getting it working by installing OCLP directly to my internal drive.
Then I tried to upgrade to Catalina but I had issues with the graphics (very laggy and definitely unusable, picture attached).
In the end I needed to make a High Sierra install drive from another Mac, and after installing now it’s working properly.
Such a pity Catalina didn’t work: any ideas?

nVDIA acceleration is broken in Mojave and Catalina.
You can only go Big Sur.

 

[amazzi44]

Good to know: do you suggest to upgrade or stay with High Sierra?

 

[Nguyen Duc Hieu]

Good to know: do you suggest to upgrade or stay with High Sierra?

If you don't have any applications which require higher OS, High Sierra is good enough.
The webcam and microphone don't work well on Big Sur in my iMac 2009.
I still keep a High Sierra volume.

 

[amazzi44]

Thank you, I’ll stay!