Did a quick google search there about this,
As long as the HDD from the sly box is a sata drive (it should be) dont see why not,would probably be best to format the sly HDD (fat 32) before fitting it to the dream 800,your box should pick it up straight away then.
Hope this helps
Sorry didnt see Ten's post on this before i put this one up,i did find one video tut on fitting a HDD into a dream 800,the drive being fitted in this was a 3.5,but there was other search results saying exactly what ten has said.