My 2011 2.3 GHz MacMini came with a 500GB HDD. But EyeTV filled it up quite fast. An external 1TB FireWire HDD didn't prove itself as reliable enough for HD Recording.
I got myself the Mac Mini Dual Hard Drive Kit and followed the instructions provided.
Worked seamlessly for me. Set up as a RAID 0 i now have a 2TB media center/PVR.
Just go for it. It was easier than i thought.