Interesting, thanks bastl, this sounds cool, although I will probably reinstall Debian, but I'll try the different options, to see what works best for me. I guess that storing the OS on unRaid gives me the benefit of safety, if a drive fails.
So I just plugged in my Synology DX513 5 x 6TB external box with an external SATA cable to the unRaid machine and all drives show up in Array Devices, this is great!