Background: I have a Late 2012 iMac which is still working perfectly; I intentionally configured it to the highest specs I could afford, for greater longevity. (27"/32GB RAM/3TB HDD/3.2GHz/NVIDIA 680MX) Those who have similar vintage hardware and who have bothered to look closely enough at Disk Utility may have noticed that the Bootcamp Assistant does some decidedly odd things on that generation of computers, whenever the hard drive is greater than 2TB. Apparently, Windows can only boot from a partition which is located entirely within the first 2TB of that drive, but Mac OS X insists on claiming the first position on the hard drive. Thus, in order to allocate all of the drive space accordingly, your drive basically ends up sliced into multiple pieces, looking something like this: |---- MacOS partition (part 1) ----| |-- Windows partition --| |---- MacOS partition (part 2) ----| I get that this is probably sub-optimal, and I get that newer computers with a more modern boot paradigms apparently don't suffer from this limitation. What I don't get, is why Mojave absolutely refuses to install, when your machine is partitioned according to this paradigm. And then, to compound the issue, Mojave offers the recommendation that you remove your Bootcamp partition if you still want to update, stating that your iMac will never again be able to use Bootcamp, afterwards. I mean... just... what the -- ?? (Background TLDR: Just read the subject line.) Potential solution: So anyway, I obviously haven't upgraded yet. I'm considering whether or not I want to try an alternate option; specifically, I'm thinking of buying an external Thunderbolt SSD, and attempting to install Mojave on that instead. In theory, I might be able to keep my existing Bootcamp on the internal drive alongside the existing High Sierra installation, as well as potentially boost my overall performance (under Mojave) with that more responsive SSD. Of course, I'm fully expecting that at a minimum I'll need to install High Sierra on the SSD first and be booting from that volume, in order to bypass the Mojave Installer's built-in Bootcamp blockade. Has anyone else had experience with similar configurations, who might be able to provide some insights on my proposed solution/workaround?