Hi. I have had this problem for ages and the way to resolve it is as follows: power the unit off. Remove the psu connector out the back. Turn the unit on with the psu disconnected. Turn it off and replace the psu connector. Turn the unit back on and the HDD should now work.
This process clears the flash (I think) and the HDD starts to work. I think the flash gets corrupted during power outages or spikes and stops the unit finding the HDD.
Hope this works, let me know how u get on.
Cheers
Springers