If you don't have the bootROM flashes upgraded, I might suggest that you just do a normal update from your previous OS. Once the flash is done, you can go back and work from the USB installer and don't have to worry about having it do the flash first. You don't even have to install the flash once it has finished the flash and go directly to the clean install. Here's the thing with the bootROM flash. This is a hardware flash and it's not dependent on the OS. Once you start the OS upgrade, it should start the flash with instructions to shut down and when restarting to hold down the on button until it gives you a funny beep, and then restarts. Hold that button longer until it actually starts to reboot. If it works, it will be a gray screen with a progress bar (or not) which you may not see, but here's how to tell if it's working and when it's finished. On a cMP, when the flash starts, it opens a DVD drawer, and closes the drawer when it's finished. (5-6 minutes?) When it's done, I believe it reboots and says you can continue your OS update. Just be prepared to wait for a long time if you are booting from USB. I recommend a SSD in drive bay 1, with the other bays empty. Install times will be shorter, but may still take several minutes to get the installation started. Essentially, the bR flash is pretty much blind, but the DVD drawer is conforting. Installation times are longer, so don't give up. I've done 3 or 4 cMPs and with the final bootup after copying the OS Install. BTW, make certain you have the full installer which is just over 6GB in the Mojave installer. Sometimes it seems to take forever, so go have a couple shots, and leave it to itself to do the initial install. Also, a clean install should only be about 30GB. I have clones of both HS and Mojave. Once you get to 144.000, You can only go back to Sierra, but you can now use NVMe drives with speeds up to 3,000GB/s. With a regular PCI NVMe card, the max is about half of that due to the PCI 2.0. I use a IO Crest NVMe enclosure card that converts PCI slot2 (X16) to PCI 3.0 speeds. It get reads of about 2,800 MB/s on a Samsung 970 EVO. That card also has 2 available slots and has a big heat sync and a small fan. (about $200) I have a 2-SATA SSD RAID0 on a Sonnet full length SATA3 PCI card. It gets reads of about 1,000MB/s and has twice the risk. You can't upgrade to Mojave without Metal video card, but once you have the OS installed, you can run it on your old GT120 video card, and you can use RAIDs. It's just the install Mojave process that's so picky.
