What kind of "Mac" are you using? What kind of hard drive are you trying to connect? Please note that only hard drives or SSDs in 2.5" format are supported and not 3.5". You need 1x USB3.0 and 1x USB2.0 for the connection and power supply. It is quite possible that this is not sufficient. Perhaps the simpler solution would be an adapter/housing with external power supply.

I only have USB 2 ports on my Mac Mini. Is that also possible?

Thank you very much for the quick feedback. I looked it up: The Seagate Barracuda 7200.12 is a 3.5 inch disc and therefore not compatible. Do they have an adapter housing in the range?