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!