Should be no problem.  There is no Dell bios or motherboard restriction on an hdd capacity.  I would make sure sata operation is set to AHCI in bios.   XPS 8940 does offer two 3.5 hdd bays where you can install two large capacity hdd. one user reported recently “My XPS 8940 now has 2 10TB WD red hard drives.” PS For a system to be able to address the maximum capacity of a data storage device of more than 2 TB (non bootable) The disk must be initialized by using GPT.  GPT partition supports full capacity of hdd.  
I have a XPS 8940 on order - it's the version that comes with a 1TB SSD and 1TB HDD. I was already not happy that there is so little expansion availability on this computer, but all of the desktops appear to be the same now. But I did find a post on the community where people have figured out how to move the empty bays so that they can get an additional 3.5" installed. But now I'm reading in the specifications that the system will only support up to 2TB hard drives. Is that true? If so, then this computer will be useless for me. Someone please tell me I'm not reading this correctly.
XPS 8940, hard drives larger than 2TB?