iMac fusion drive Mojave upgrade - com.apple.diskManagement errror 69854

[cameronjpu]

I just tried to upgrade my iMac 2014 with Fusion drive, and I'm getting this error. Of course, I don't have a backup. The previous OS was 10.10.

I can make myself a high sierra USB drive, will I be able to use that to get this thing fixed? If so, how? Do I boot from that USB disk and install the OS? If I do that, will it upgrade the 10.10 to 10.12 and keep all the data? Or will it be a clean install?

Thanks for any help!

 

[cameronjpu]

Update for anyone reading this in the future. Booting from High Sierra installation disk worked like a charm. I was able to install High Sierra as an upgrade to El Capitan and it worked just fine. Thankfully that was an option because the system was so out of date.