Ok it uses a SATA drive.
So you can use either SATA 1 or SATA 11 but it will only run the SATA 11 at SATA 1 speeds ie 150mbps instead of the 300mbps.
From what I can see it will only take one internal hard drive so if you want to increase the storage capacity you will have to replace the original hard drive or use an external USB hard drive if you want to keep the original drive.
I came across this review :
http://www.pcpro.co.uk/labs/83639/acer-veriton-3700gx.htmlWhich says :
It's worth noting that the Acer isn't the easiest to work on either. Three thumbscrews have to be unscrewed before the case comes off. All three drive bays are part of one assembly, which pivots and lifts out in one. Cabling is a mess, though, with plugs and wires behind the optical drive restricting access to the memory slots.
The hard disk is difficult to remove, and we also discovered that the edges of the steel cage are razor-sharp.