The 500mA is for USB2 (max of 5 loads of 100mA), this drive is USB3 (max of 6 loads of 150mA = 900mA).
External drives often have a (Y or chained) cable that connects to 2 USB ports to get the power needed. Check the specifications of that drive very well before buying anything.
A second reason why that adapter is too tight fit is the surge current (inrush current), a lot of power (relative) is needed to get the disk spinning inside, his is only for a short moment, but in this short moment the current is high, the voltage level might collapse to such a low level that the RPi starts to act weird, or just reboots.
The most safe option is to power the RPi with a separate supply and connect the drive through a powered USB Hub. This ensures that the RPi will not have to deal with any effects from power fluctuations from external devices.