Best answer: USB-C and Thunderbolt may use the same port, but they are by no means the same connector. There are some key differences that prevent most Thunderbolt 3 products from connecting to USB-C ports.
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Two protocols, one port
USB-C and Thunderbolt 3 both utilize the same reversible port, which can lead to a lot of the confusion surrounding the two protocols. While a USB-C port and Thunderbolt 3 port look the same, Thunderbolt 3 has some extra hardware compared to the more widely-adopted USB-C.
It's important to know, too, that your USB-C accessories and cables will work with a Thunderbolt 3 port. All Thunderbolt 3 ports are also USB-C ports. However, the same cannot be said for the other way around; not all USB-C ports are also Thunderbolt 3 ports.
A story of PCIe
Thunderbolt 3 was developed by Intel and offers high-bandwidth data transfers between devices, with maximum transfer rates of 40Gbps (or 5GB/s) available. Importantly, it's also built on PCI Express (PCIe) and DisplayPort, and this goes to why Thunderbolt 3 cables and accessories aren't compatible with USB-C only ports (for the most part; more on that in a bit).
Thunderbolt 3 uses PCIe, which offers more data throughput.
PCIe offers a lot of data throughput — it uses multiple lanes to move large amounts of data very quickly. This is what gives Thunderbolt 3 its speed, but it means that if you want to use a Thunderbolt 3 accessory, you need to plug it into a Thunderbolt 3 port.
Because USB-C doesn't use PCIe. USB-C is merely the latest generation of the USB standard, something that's been present in everything from computers to cameras to phones for a while now. So when you try to plug in a Thunderbolt 3 accessory to a USB-C port, it just won't work because there's no PCIe controller in the USB-C port.
While you can't use most Thunderbolt 3 devices with a USB-C port, the same isn't true the other way around. You can plug a USB-C device and cable into a Thunderbolt 3 port whenever you want because Thunderbolt 3 fully supports USB-C.
So a USB-C hard drive you have can be used in any of the Thunderbolt 3 ports on your MacBook Pro, or in the USB-C port on your 12-inch MacBook.
It's 2018, so of course, it gets a little more confusing
While its true that most Thunderbolt 3 accessories won't play nice with USB-C ports, a new wrinkle was added in early 2018. Intel introduced a new, mid-generation controller family for Thunderbolt 3 called Titan Ridge. While Titan Ridge maintains the 40Gbps throughput of Thunderbolt 3, it adds the ability for accessories using this controller to have a USB-C fallback mode.
All Thunderbolt 3 ports are also USB-C ports, but not all USB-C ports are Thunderbolt 3 ports.
This fallback mode means that accessories with a Titan Ridge Thunderbolt 3 controller will be able to fall back to using USB-C when plugged into a USB-C port or hub. While you won't get the same speeds and other capabilities as you would if your accessory was plugged into a Thunderbolt 3 port, it's better than nothing.
Unfortunately, nobody really advertises what kind of Thunderbolt 3 controller they use in their devices. So you'll need to look out for devices advertising something along the lines of "connects to Thunderbolt 3 and USB-C." As with a lot of technology, nothing's a sure bet, so be sure to try and use the correct ports for your Thunderbolt 3 accessories.
Daisy chain away
Another advantage that Thunderbolt 3 has over USB-C is daisy chaining. You can connect up to six peripherals to a single host Thunderbolt 3 port by chaining them together serially.
For instance, if you have a bank of Thunderbolt 3 external hard drives, you can connect one to your Thunderbolt 3-equipped Mac, then connect a second hard drive to the first drive, a third to the second, and so on. All of those drives (again, up to six) will now be available for use by your Mac.
While devices like USB-C monitors can act as hubs for other devices, you can't yet connect device to device to device serially as you can with Thunderbolt 3.
Get every port you might need
If you have a Thunderbolt 3-heavy computer like recent MacBook Pro or Mac mini models, this hub will offer up any port or slot you might be missing, including four USB-A ports, Ethernet, and an SD slot, all through a single Thunderbolt 3 connection.
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