If your Mac has USB3 then it also has a thunderbolt port so you do not need an adaptor for the Mac.
What says you can use Thunderbolt with an adaptor? The Seagate Backup Plus drive?
If you are talking about using the Seagate drive with Thunderbolt instead of USB 3 then you buy an adaptor for the drive itself.
If you look at the end of the drive where your USB 3 cable connects to the drive you should notice that the end portion of the drive can be removed.
This allows you to change that specific drive to use what ever interface you want to use, from USB 2, USB3, FireWire and Thunderbolt.
From the Seagate Backup Plus website you can click on the Accessories tab and it lists all of the available adaptors for your drive.
http://www.seagate.com/external-hard-drives/portable-hard-drives/standard/backup-plus/
Unless you just want to spend more money just to spend it, I personally do not see ANY reason to go with Thunderbolt over USB3 on a portable non-SSD hard drive. The drive is not going to transfer more than 120MB/s regardless of the interface.
USB3 can handle "640 MB/s" and Thunderbolt can handle 1280 MB/s but changing from USB 3 to Thunderbolt is not going to make the hard drive itself perform at a faster speed just because the technology allows for faster speeds.
The hard drive is the limiting factor.
This may not be your case and you may completely understand both interfaces but I have lots of friends that hear USB3 is slower than Thunderbolt so they want to buy everything Thunderbolt even though in a majority of their cases it makes no difference at all also the lower end adaptors and hard drives remove the ability to daisy chain Thunderbolt devices which is another major Thunderbolt selling point.
