The front top position is for 3.5. I moved mine from the front to the front top position (super easy). The rear top is 2.5 (you can fit two in there).   Page 12 of the service manual. 4 and 9 are the 3.5 HD cages. The riveted unnumbered one at the top back is 2.5
My new XPS 8940 has a 256 GB SSD and a 2 TB 3.5" drive. I would like to add a 2nd 3.5" Hard Drive to the enclosure. The chassis currently has a couple of unused Hard Drive cages. One of them is 2.5" and appears to be riveted to the chassis. The other unused cage is slightly bigger and held in place with screws, but still looks to be too small to fit a 3.5" hard drive.  Is there any way to replace one of the smaller unused hard drive cages with a 3.5" cage?  
XPS 8940, add 2nd 3.5" HDD