That's correct bud, if you flip it over, you should be able to see where the second HDD would go. If you look at the laptop as you're using it normally, the 2nd drive goes in the bottom left hand corner (where your palm would rest) so if you turn it over and look in that general area, you'll see the slot.
You can put a second HDD in there, however, Acer have put a weird cover on the bay which means if you wanted a second drive, you'll need to order the correct cover from Acer. I've heard of some people actually grinding the original bay down for it to fit, but I can't comment how well this works.
If you give Acer a call (the UK number is 0870 853 1005 iirc), they can sell you the cover you require.