Figured I'd post my experience of caveats, trials, and tribulations in getting an upgraded 2010 Mac Pro from blank drive to Mojave for anyone else who this might help. The machine had been running El Capitan for a while, and I hadnt been using it much. I had updated the CPU a long time ago (I needed virtualization extensions for Docker, also got an extra 2 cores out of the upgrade ) and added RAM, neither of which had any impact on this so I'm going to leave those off. I started with a newer, larger SSD than I had before (250GB Samsung 850 I had lying around) and a PCI-E --> SATA 3 (6gbs) card to mount it on instead of in one of the bays. I picked up an NVIDIA GTX 680 (EVGA ref model I found on ebay), flashed properly (there's about 50 threads on that process here so I'm not going to delve into it). The GTX 680, when flashed, unlike a lot of the metal capable cards that you can use with Mojave has boot screens, which I'll come back to in a sec. I tried installing High Sierra directly first, found out that the machine needed a firmware update and for some reason the shutdown button to do it remained grayed out. I installed El Capitan first on the (it turns out correct) assumption that using the installer inside a booted OS instead of booting into the installer might help. I suspect Sierra would be fine too, but I had an El Capitan installer handy but not a Sierra one. Because of the firmware req using an install from another machine or target disk mode wasnt an option. Installing El Capitan went fine, but when attempting to install High Sierra ran into the snag that the card the SSD was sitting on shows as an "external" drive to the OS and HS claims it cant install without an EFI partition. If you go to a faster pcie SATA option as I did you may run into this or not depending on the card. Temporarily moved the SSD into a drive bay, ran the installer, firmware updated, HS installed. Repeated the process with Mojave, another firmware update and the full install later and mojave was running. Now I tried to enable Filevault (because hey, I have boot screens) and discovered that Apple has a check that stops you from enabling it on a 5,1 in Mojave, even if you have a card that has boot screens. Rather than directly work around that check I shut the machine down, booted it in target disk mode, booted my laptop off that, enabled filevault, rebooted the mac pro. Viola, filevault! Moved the SSD back to its 6gbs card, enabled trim via trimforce: fully working Mojave install, faster SSD, filevault and all, on a 5,1. Dont know if this will help anyone else, but I hope so.