It is best to make 2-3 partitions on it since it will prevent the whole 1.5TB drive from being fragmented all over and then taking days to defragment... But for that to work best, you have to separate some files from others:
- Music, video and image files don't fragment at all (if you don't move them all days of the week), and will never require to defragment, ever.
- Games and softwares can fragment a bit, and you will need to defragment that partition a few times a year.
- Swap and Windows files does fragment A LOT!! So you will need to defragment it once a week.
For that reason 3 partitions is best, one for each of these 3 categories of files. Then only the Windows partition will need to be defragmented all the time.