Hello everyone, I’m new here and at my wit’s end.

I collect and restore Apple computers. I upgrade them as much as possible and install the appropriate macOS using OpenCore to keep them usable today.

Specifically, I’m working on an iMac Late 2015 (Model 16,2) with 16 GB RAM. I have two SSDs installed: a standard SATA SSD and another one in the M.2 slot via an adapter. The M.2 SSD is recognized, and everything seems normal so far. Since the iMac natively supports macOS Monterey, I wanted to install Sequoia 15.3 or 15.2. I’ve successfully installed 15.2 on multiple older Macs without any issues.

The Problem:

The installation does not work reliably on this iMac. Normally, my process is:

1. Download macOS through the app,
2. prepare a USB stick,
3. boot from the USB stick,
4. format the target drive in the Recovery menu, and
5. start the installation.

This has always worked flawlessly – except on this iMac. Either the installation does not start automatically, or it gets stuck. By forcing multiple restarts, I can eventually get macOS 15.2 installed, but if I unplug the iMac for an extended period, it refuses to boot from the M.2 SSD or freezes.

If I install macOS on the SATA SSD, it runs fine, but this whole behavior still seems odd.

Possible Causes?

• Is this a bug?
• Are additional OpenCore settings required?
• OpenCore is set up correctly, and NVMe support is enabled.
• After a crash, I only get the standard error message ("Your computer restarted because of a problem"), but no crash logs appear in the macOS Console. but a Panic? (https://pastebin.com/raw/zHVth22g)

Interestingly, Monterey runs without issues, both during installation and in daily use. I’ve thoroughly checked the hardware:
✔ Apple diagnostics show no issues
✔ Both SSDs are new
✔ Thermal paste and PRAM battery have been replaced and tested
✔ PRAM reset performed

A hardware defect is therefore highly unlikely. The issue seems to be related to OpenCore or Sequoia.

Additional Observations:

• When booting Sequoia from the USB stick, the progress bar freezes, and all USB ports seem to turn off (mouse lighting goes dark).
• I have to press the power button three times before the installer finally loads into Recovery Mode.
• During installation (when about 10 minutes remain), the iMac should restart multiple times to continue setup but freezes instead.
• I have to manually intervene to get the installation to complete.

Has anyone experienced similar issues or found a solution? 🤔