I assume D: is a separate physical drive and not a partition on a single HDD?
With your experiences it is surely a matter of how far do you want to research this. The trouble seems to be limited to D: only which makes Megatyte's idea a must - to begin with.
After that it is posible(just), that the IDE controller or cable is faulty, but with an 8Gb disk there is a limit to how hard you should try. I wouldn't be happy with what seems to be a random scrambling of the disk read/write ability, and would consider binning it.