Thunderbolt 3 to Thunderbolt 2 Adapter
This adapter works perfectly with my older Thunderbolt external drive. I have not had a single issue with it.
Hopefully I can get a new SSD Thunderbolt 3 drive to replace my current external drive so I won't need to use these adapters, but for the time being, this works fine!
The adapter is $29, you can purchase it from the Apple Store: