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.
IMHO partitioning is useful if you want to separate Windows OS files from your data files - this allows you to perform image backups of the OS without having to include your data files, and file-based backups of the data without including the OS. It also makes it easier to upgrade the OS without affecting your data.
But for a drive that contains nothing but data, generally speaking I see no useful reason to partition the drive. You can use folders to organize your data into different categories more easily than you can with partitions. Splitting the drive into partitions will actually increase head movement if you're doing a lot of access to different categories of files at the same time. And if you want to move a file from one category to another, the system will have to read and write the entire file rather than just moving it's directory entry from one folder to another.
So if this is just a data drive, my recommendation would be to use a single partition unless you have some particular reason not to.
I agree with sminlal, the index only has to change the sector location in order to move files around on a single partition, whereas it would take time with partitions... so you just slowed yourself down. And while fragmentation can be a problem if you're moving files around all the time, it should take a while for there to be any significant performance hit on a drive that size until you've got most of it filled, in which case you should just start a defrag before you go to bed one night and wake up to find it done. Defragging isn't really an inconvenience when it's planned/scheduled