Harmy said:

So the computer is up and running but I don't have legally activated Windows yet, because I ordered it slightly cheaper from a different company and it hasn't arrived yet - but hopefully it should arrive today or tomorrow at the latest. Meanwhile, now would be ideal time to test the HDDs but I really don't know how to go about it.

That's not a problem, you can download official ISO of your version from Microsoft and install it now, you have like 30 days to enter product key.