We’re homeschooling this year so I’m now debating seeing if I can fix our old iMac. It stopped booting up recently and so I was looking to just reinstall macOS. Unfortunately, that isn’t even working at the moment. I’ve included a few screen shots that may give you some insight. Note that only one drive shows up now (it used to show a second one).


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Looks like the OS installer is not seeing the drive correctly. Couple of things to try.

Booting the computer in target disk mode and erasing it from another computer.
Boot the computer from a different computer or external drive that has an OS installed then try again to wipe from that drive.
Check the ribbon cable to the internal drive (which requires taking the computer apart). Depends on how brave you are.

The computer's drive may have died (which is extremely common on older 2015 iMacs due to the crappy drives Apple installed in them (and still does if you opt for the cheaper version).

I'm no longer doing hard drive replacements but depending on the year of the iMac, you could probably yourself. If it's the thin iMac, forget it. If it's the thicker iMac, then it's not too hard.