Thanks for all the advice guys.
I am really hoping that the drive isnt broken. I bought it new a couple of months ago after the previous system drive failed. I don't want to go through the whole reinstalling WHS thing again.
My server is a HPMediasmart server, and the drives kinda click into the sata connectors at the back when you install them, so if one of them is dodgy I can't imagine how I'd replace it.
At the moment, all the Delayed write errors seem to be coming from paths beginning with c:, or occasionally d:/. Both volumes belong to the same physical disk (the system drive) so I'm hoping the problem is just localised to there. I am nervous though because the d: partition has the "Shares" folder which points to the combined storage pool of the other 3 drives. When the server fails to write to the MC library (located in a Share), the error isn't pointing to the UNC network path, but the local "d:/shares/..../mc17 library". So even though the other drives might be fine, the fact that the server is failing to write data to them properly as it tries to go through the local path is, as jgreen mentioned, likely to cause trouble all over the place.
I will do as you suggested and try to find the maker's disk checker.