2Tb.
Edit - though if it's the version with less on-board RAM (i.e. that was originally 30gb) it might struggle a bit if you use all that space and so have a vast number of files. And that's all assuming you use RB not the original firmware (which has some fairly low-limit on the number of files it can cope with).
5th gens don't have the 128Gb limit of the 6th gens though.