I bought an expensive Dell Precision 7510 with two hard drives. When I went to install Fedora 25 it couldn't find the drives. I read somewhere about changing the SATA configuration in the BIOS to AHCI, and that worked. However, then I had screwed up my Windows which wanted it set to RAID, so that took some extra time to put right.

All good now but, I still wonder why Fedora 25 can't handle the SATA configuration set to RAID. What's the problem?

Not Fedora, but the Linux kernel. Go search for "lenovo block linux install" or similar from late last year.
I saw a thread on LKML where the devs were arguing whether Linux should accommodate crap hardware implementations - even new ones. I think the patches still haven't been accepted. The "fix" is as you did - use ACHI.

SATA RAID should just about be an oxymoron. Raid always statistically increases a drive's chance of failure. Combine that with consumer grade drives and you have a recipe for disaster, especially when running 4 or more.

So, motherboard makers are wasting our time and money with these implementations. ACHI, on the other hand is useful. The extended functionality allows things like running SSDs in "performance mode".

If I were you I'd just use rsync to periodically backup folders from one hdd to the other. It's far less stressful than using them as mirrors. If you have a real concern of redundancy, use cloud storage where things are usually stored on an actual SAN. If you have privacy concerns about that, use p7zip or something like it and hard-key encrypt the stuff.