Thank-you @michaelsmoody. So apart from swapping the red and black, the NAS can still monitor, correctly, the speed of the fan with this particular fan? The DL4100 is not under any warranty and I have had it open so taking it apart and purring it back together is not a problem.
In my experience there are two things can can go bad and cause grief. First one is the PSU and for that I have a comparable spare that has the right sized plug and the exact voltage with more than enough amps. An actual ACER brand laptop PSU.
The second thing that can end-up going is the fan. The fan in mine is still running fine and where the NAS is there is very little dust, but I would like to be prepared and have a spare fan in-case I end-up getting fan error codes.
Do you have a technical specifications for this FAN?