I'm using the Western Digital Elements drive, 2TB, which was the largest of the recommended drives available at the time.. I think it cost about $80 or so at the time.
WD Elements 10B8 1012 (2000 GB)
It's been a reliable drive for me, with over a year of operation on it now. With my usage patterns, which are to record/watch/delete for the most part, I rarely if ever exceed 10% of its capacity. There are very few shows that I'd want to save, and if I did, I'd pull them off of Tablo with CycleJ's Tablo Ripper and archive them on to my NAS drive.