I've just run the Hardware & Devices Troubleshooter and it reported "Troubleshooting couldn't identify the problem".
The PC was self-built 6 years ago, using an Asus P8H61-MX motherboard. It's run without issue for 6 years, initially with a small SSD as the C: drive and a 2.5" 120Gb HDD for data (D:), and over the years I've upgraded the C: drive SSD to 250Gb and the data drive to 1Tb 3.5". This failed after 6 months, and while waiting for the warranty replacement from Seagate, I bought a WD 1Tb drive. That is (still) working fine. Having got the replacement from Seagate, I thought I'd set up a Storage Pool for disk mirroring of my data drive - and that's when I found that it wouldn't let me configure the Seagate drive, even though it was visible in the BIOS on my PC - and visible at the Windows level for configuration in Disk Management when installed in a different PC.
I've also tried all the obvious things like swapping leads and SATA ports in my PC, and in all cases it can see the WD drive at Windows level, but never the Seagate one, even though that is visible to Windows when plugged into another PC.
I think I'm stuck! Any help appreciated. Martin