Copy to same drive or separate drive?
Copying data to a different drive negates the risk of losing all your data due to physical failure of your primary drive (as would be the case if you merely partitioned it and used the other partition as a backup). 

Since that’s the principle cause of data loss (way easier to repair a corrupted partition scheme than a broken HDD/SSD), it makes sense that copying data to a different physical device is the most effective method of backing up — it’s the only one in which true redundancy exists.