First and most important you need to determine which protocol your external drive uses. USB-C is a connector style and not a connection protocol. Most external drives using a USB-C connector are likely only using the USB3 protocol. I know this is can be very confusing since I'm a firm believer the connector style should have been given a different name to minimize confusion.
If the external drive uses the USB3 protocol, then you will need a USB-C to USB-A Adapter.
If the external drive uses the Thunderbolt protocol, then you will need to find a USB-C Thunderbolt to Thunderbolt 2 Adapter/Cable. What you need depends on whether you are connecting to the ports on the back of the external drive or whether the external drive has a built-in USB-C cable that you need to interface with. There are cables/adapters out there, but it requires making sure you have the correct male/female connectors on those cables/adapters.
If the external drive only uses the USB protocol, then you cannot connect the drive to a Thunderbolt port as the protocols are not the same.