It looks like the unit will take most standard 3.5" IDE HDD. Having just replaced mine with a 250GB model. After suffering the same problem. Easy enough, need nothing more than a philips driver and roughly 10 mins.
To access the hard drive. Remove the 6 retaining screws from the top of the case. 2 at the side, 4 across the back of the unit, and remove the top. Then unplug the ribbon cable and power cable from the HDD.
Flip the unit over and remove the 4 screws holding the hard drive brackets in place. You can now remove the HDD assembly. Remove the brackets from the HDD and attach them to your replacement drive.
then reverse the process to rebuild it.
Once completely re-assembled, power up the unit. You'll receive notification that the HDD requires formatting. Just click ok, takes a couple of mins and then you should be up and running.
Dave
July 2011