🔧 Solution Guide: Fixing Boot Issues on iMac 21.5 (2010) with macOS 11–15 (Big Sur–Sequoia)

This guide addresses boot failures, fan ramp-up, and NVRAM-related problems on a modified iMac 21.5-inch (Mid-2010) with an Intel i3 processor and an NVIDIA Quadro K2100M GPU running macOS versions beyond official support.

⚠️ Background Context

Your iMac 2010 is not officially supported beyond macOS High Sierra, so running Big Sur or later requires OpenCore or similar bootloader patches, especially for unsupported GPUs like the K2100M. The symptoms you described—black screen, max fan, non-working keyboard—are usually linked to firmware/NVRAM issues or a misconfigured OpenCore EFI.

🛠 Step-by-Step Fix Guide

🔹 Step 1: Ensure You Are Using OpenCore (Not Clover)

• If you're using Clover, switch to OpenCore, as it provides better support and stability for legacy Macs.

• Follow Dortania's OpenCore Legacy Patcher (OCLP) guide:
  https://Dortania.github.io/OpenCore-Legacy-Patcher/

🔹 Step 2: Rebuild and Clean OpenCore EFI for Sequoia

1. On High Sierra, download OpenCore Legacy Patcher (latest version).

2. Use it to build a fresh OpenCore EFI for your machine:

   • Select model: iMac11,2

   • GPU: NVIDIA Quadro K2100M (Kepler)

3. Copy the new EFI folder to your boot USB stick or EFI partition.

✅ Ensure these settings:

• NVRAM > Add > 4D1...UUID > boot-args includes:
  keepsyms=1 debug=0x100 -v

• Misc > Security > SecureBootModel is set to Disabled

• NVRAM > Delete includes boot-args, csr-active-config, nvda_drv, etc.

• SMBIOS is set to a supported model (e.g., iMac12,2)

🔹 Step 3: Patch System After Installation

• Boot into Sequoia or Big Sur using OpenCore.

• Open OpenCore Legacy Patcher.

• Select Post-Install Root Patch to patch:

  • Legacy GPU acceleration (if needed)

  • Any system kexts required for your GPU or board

🔹 Step 4: Fix NVRAM Lock Issue

Sometimes, old Macs with modified hardware or NVRAM corruption can’t store settings properly.

1. Reset SMC:

   • Unplug iMac, wait 15 seconds, plug in while holding Shift + Control + Option + Power for 10 seconds.

2. Manually Delete NVRAM Entries (optional):

   • In OpenCore, press Space at boot menu and select Reset NVRAM

   • Reboot and enter macOS, reapply patches if necessary

🔹 Step 5: Disable hibernation and power saving (can cause boot bugs)

In Sequoia:

bash
CopyEdit
sudo pmset -a hibernatemode 0
sudo rm /var/vm/sleepimage
sudo pmset -a standby 0

Also, disable any “Power Nap” or “Automatic restart” settings in System Settings > Energy Saver.

🔹 Step 6: Test Reboot and NVRAM Persistence

• Test cold boot and warm boot several times.

• If problem persists:

  • Check if nvram -p in Terminal shows retained values.

  • You may need to replace the PRAM battery (rare but possible, or simulate via capacitor solder).

🔹 Step 7: Optional GPU Switch (Last Resort)

Even though Kepler is mostly supported up to Sequoia, sometimes GPU patches or hardware quirks break things.

If none of the above works:

• Try using iMac's iGPU (HD Graphics) by removing the discrete GPU

• Or, test another known-compatible Kepler GPU like GT 710

✅ Conclusion & Notes

If you’ve followed all steps and it still fails to boot consistently, it’s very likely related to corrupted EFI, faulty NVRAM, or an edge case of unsupported firmware interaction. Let me know if you want a review of your OpenCore EFI folder — you can upload it for inspection.