Software RAID is used to implement the various RAID levels in the kernel block device code. I hope this is of help to someone.This guide focuses on setting up Software RAID on Debian 10. There are cheaper products, using the same Marvell chipset, but they are not, AFAIK, ISO 9001 certified so quality is less assured.
I'm pretty pleased with it and it is a great improvement compared to the Windows 10 software RAID solutions.
Now here is the problem, if indeed it is a problem, having moved the PC/NAS the two drives have taken the best part of a week to resync. Well, with the cold weather I have decided to bring the PC/NAS into the main house as the hard drives were often running too cold. The NAS side of things has 2x4TB WD Reds running in RAID 1 configuration with backup to a separate machine on the network. As people here may (or may not) recall, earlier in the year, I re-purposed a low energy consumption PC to be a combined PC, for use in the garage and a NAS for data storage/media server.