Why won't my iMac update?

Post date: 2020-03-27 03:49:47

I'm having problems getting an OS update on my iMac and have no idea why.

This all started when I realized a new piece of software needed OS X 10.13 and I was like, "oh wait that's weird I'm still on 10.12.something." I know, you would think I would have noticed by now. I assumed automatic updates were on. Also I apologize in advance but the computer in question is turned off at the moment (house shuffling, COVID-19, work-from-home orders, and so on) so I can't answer specific questions about hardware. The iMac is from 2009 or 2010 and I did read on Apple's site that the hardware should have no issue running OS X 10.15.

I went to the App Store and searched for 'update' or 'os x' or 'Catalina' but it said "no updates available" so I went the brute force route and found the .dmg for the most recent version of 10.15 on Apple's site. When I downloaded it and ran it I got the message, paraphrased, "You can't install 10.15.2 from this version of the OS, you need a previous version of 10.15" so I said FINE and went to download the earliest version of 10.15 that I could find. When I ran that one it said "You can't install 10.15.1 from this version of the OS, you need at least 10.15." and it stopped there, no sub-version after the dot.

I thought this was odd so I tried 10.13 and got the same message of "You can't install 10.13.1 from this version of the OS, you need at least 10.13." and it stopped there, no sub-version after the dot.

I would strongly prefer to update my OS somehow without reinstalling everything. I do have a daily Time Machine backup of everything though so if God forbid I have to wipe and reinstall I can do that.

Questions:
1.) Do you know a way to fix this issue of it saying it can't update my OS from a disk image that I downloaded directly from Apple?
2.) If I have to wipe and reinstall, can I just move programs over from my Time Machine backup? I have some old software with long-lost product registration keys and if I can't just drag them over to the new clean install then I am losing them forever (as far as I know).
3.) If I have to wipe and reinstall, can I do that from my existing HD? Or do I need to dig out my optical drive and burn an install DVD? I haven't done anything like this in over a decade; I'm a bit behind the times here.

Any thoughts you have are welcome, and I'll be hooking everything back up in the next few days so I can try whatever you all come up with.

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