I'd go with Ogelthorpe's lead. The AHT does not test storage media capabilities, just stuff like the RAM, CPU, GPU, video, etc. Hence, the HDD and optical drive are just checked as "present" (i.e., the controller card on the things answers back when pinged by the test), but not checked to see of they mechanically work.
Other places to look for spare parts (and how to put them in) are PowerBookMedic.com and IFixIt.com.
You could also take it to the Apple Store and the Genius Bar, they will take a look at it for your for free.