gunt76 raises a good question, although I can tell you that upgrading to a new HDD is an investment in time. At least 20 hours of time (or more). So, I'd say if you need a quick solution, buy the WD drive that is of equivalent size or larger and put it in the device. It will be a quick operation and it will be back up with little or no effort. BTW - don't forget to go and format the new drive. from the web interface. :-)
If you decide to upgrade the HDD's , I'd recommend reading around and make sure you find a solution you're comfortable with. Not everyone is OK with using DD and copying partitions, and the like. I'd recommend to start reading here:
P.S. I'm also working on writing up another way of upgrading since I have just completed this process myself. I call it "The Easier Way" method. Well, read up and you be the judge.
Let us know what you decide and how it turns out.