iMac: Wrong drive, black screen, lightning bolt

Sounds like the iMac is starting up in Target Disk Mode.

Transfer files between two Mac computers using target disk mode - Apple Support

Try changing the Startup Disk.

Change your Mac startup disk - Apple Support

Is this something like what you're seeing?

If it is that's the screen for Target Disk Mode which is achieved by holding down the T key while booting.

If this is to what you've seeing try to get a photo of the screen and post it with your replay along with details about your Mac, system according to this user tip:     Writing an Effective Apple Support Communities Question - Apple Community

👍 I'm glad to hear that fixed your issue.

peazer wrote:

That’s exactly what I’m seeing. I just followed your advice & all is well.

Thanks so much!

David

That's good news. You're welcome. 👍