4 stars because there are no instructions included with the product. It plugged in and came up properly (and proceded to set up for "Time Machine". ) It came up quite quickly (45 minutes) instead of the 2 hours it took with my HD drives which lasted only 1 to 2 years each. I have observed that "Time Machine" runs at least twice as fast as the previous disk allowed.
If it lasts two years, I will be happy with the purchase.
Looks great on the base of the iMac.