According to the specs sheet listed above from Acoustic Sounds, it has USB 2.0 input for external disk. Should not be a problem using an HDD, load it up as much as you want, I may do the same thing at first since I'm not too great at understanding the networking aspect myself (only recently got a NAS and it seems to act a bit wonky sometimes). I would still like to see some reviews of this thing first before buying it as I am torn between this or just buying a Macbook Pro, which can obviously be used for a lot more, but still makes a pretty nice music server. The initial impressions I've read from Computer Audiophile seem to suggest the Lightning app is not up to par yet either, which is a pretty big concern.
HDD is NOT supported just yet, only NAS at this point.