Booting iMac with Option key = kernel panic

My friend has a Late 2015 21.5-inch 4K iMac. It shipped with a HDD, and he was pulling his hair out with performance issues. He bought an external Samsung T7 SSD and installed Big Sur on that, and his performance issues went away.

But now, he cannot use the Option key to choose his startup disk on the fly. If he uses the Option key, no matter which drive he chooses, it will kernel panic and reboot using the other drive.

On the other hand, if he selects a startup disk using System Preferences, it will boot without a problem. He’s worried that there might be a problem down the road when he decides to wipe the HDD. I did tell him that since the SSD is booting reliably when he selects it with System Preferences, he should be fine, but I also told him I would post here and see if there were any other suggestions from the community. It does appear that the HDD lacks a recovery partition.

I’ve never seen a situation where the method of selection appears to determine whether the computer will boot successfully or kernel panic. It seems to be clear that this is not a problem with a specific drive or installation, but I’ve no idea how to fix this. Thanks in advance!

Hi William. I’ve been watching your post as I have a ssimilar iMac using an external SSD for operation
iMac.pdf (298.6 KB)

Just now I did an Option Restart and did NOT get a kernel panic. I was offered several drives to choose among

So I assume it is his computer.
My iMac in smaller and newer


I have an even older iMac which has no problem booting from an external SSD.

  1. Does the iMac boot without error to the HDD if he disconnects the external SSD (and any other external drives) and boots with the Option key held down?
  • If so, then perhaps there is a problem with the external SSD (or other external drive).
  • If not, then perhaps there is a problem with the internal HDD.

By “problem with” I don’t necessarily mean a hardware problem: there could be a problem with macOS on the drive, or the the formatting of the drive.

  1. Similarly, does the iMac boot without error to the HDD if there is a different SSD connected?

It doesn’t seem to matter which drives are connected. I could probably have stated this much more simply:

  • If he selects any drive via the Startup Disk pane in System Preferences, it will boot without problem.

  • If he uses the Option key to select any drive, it will fail to boot and give a kernel panic.

I’ve never seen such strange behavior. My apologies to anyone who was confused by the original post!

That happens even if there is only one volume listed (the internal HDD)?

My gut feeling here is telling me that your Recovery partition is messed up. Unfortunately, I don’t know of a procedure to reinstall it.

I’ve seen perhaps-related symptoms with the Startup Manager (hold option at start to choose startup disk) before, although on earlier machines.

I don’t have notes, but I think what I experienced was: held option at start, expecting to see the Startup Manager, but instead a grey screen was shown for longer than usual, followed by the Mac startup up from the previously configured disk (and then a little later, I think the boot process hung). This prevented me from booting from an external drive to troubleshoot the issue.

I think I’ve seen this situation at least twice, and eventually resolved it by opening the machine and disconnecting the internal drive.

My suspicion in these cases was that a faulty internal drive was causing Startup Manager to silently fail.

So I would suggest disconnecting the internal drive & seeing if that helps matters. Of course this isn’t easy. Instructions here - iFixit rate this as “moderate” difficulty: iMac Intel 21.5" Retina 4K Display Hard Drive Replacement - iFixit Repair Guide. So probably this is a last resort.


That’s a good question. I wouldn’t have thought to try it with just the one drive. But we may never know, since he just told me he has already wiped the internal drive. I doubt he’d feel like humoring me by installing a system on it to find the answer.