Thanks but I think I'm too paranoid to hack my own firmware. The OP's non-destructive disk based methods seem safer, but I'm just curious about what maintenance would be required with the foxfoobar or Next Loader methods over the long run. The Fusion Drive approach is attractive as it uses core OS features only, would be understood by future OS updaters, less maintenance required by me. Unattractive as it's more risky like a RAID 0, if either drive fails the whole Fusion drive is dead. Also can't completely control if data lives on the faster or slower drive. Can't find much good information on minimum Fusion drive partition sizes online, might just have to tinker around with it. What I'd like is the inverse of what Fusion Drive is designed for - a small HDD partition combined with a large SSD. To force most of the drive's data to live on the fast SSD. Just thinking out loud... ideally I could make a small partition on the HDD and a small partition on the SSD, make that a Fusion Drive for booting, OS and apps. Those are things that would be nice if they ran fast generally, but performance is not mission critical. Then have the remainder of the SSD partitioned as a pure SSD where I could keep video editing projects and scratch files, stuff that needs to be fast always. And the remainder of the HDD partitioned for longer term media / data storage that doesn't need the speed. Also would put recovery partitions on the HDD in case something goes south with the NVMe drive.