I'm going to go ahead and be late to the party here and go way back to post #1. Probably for the best. ;)
I don't need FreeNAS for anything besides managing the Plex storage drives. I still plan to run Plex in Windows, which means those drives need to be available in the Windows environment.
The quickest solution here would be to use the regular Windows disk management to set up a "striped with parity" volume on dynamic disks (RAID5) - this would make the data locally visible to your running Plex instance without the overhead of having everything loop through a virtual NIC into a hypervisor and then through the HBA and on. Performance will definitely be better doing this, and because it's the "legacy" software RAID rather than Storage Spaces, Microsoft is less likely to break it with every new Windows 10 feature/security update.
Regarding the HBA and Hyper-V; Windows 10 Pro doesn't officially support DDA (direct device assignment) so that's why it's being an absolute bear to enable. I believe I read something about it having been hacked into functionality but that was specifically for users passing in GPUs for virtual gaming instances, not an HBA for storage - and the interrupts might hook in differently.
As prickly as some of the other advice you've gotten is, it's not entirely off-base though. FreeNAS and other storage solutions, whether it's unRAID, DrivePool, or Storage Spaces, are intended to be run on a more stable software platform - whether that's a Windows Server distribution or a Linux/BSD on bare-metal, consumer operating systems do tend to be a little more fragile than the others.
A single-bit error in memory during a ZFS scrub has the potential to destroy all your data.
If you know a way to ensure
an AES256 a Fletcher4 hash collision (which is what's necessary to make this true) please let me know - I have a Powerball I'd like to win.