USB-C introduced simplicity: a simple flat connector with rounded sides that has two correct orientations: the long side can be inserted into a compatible port without checking. It’s used universally for USB 3, DisplayPort, and Thunderbolt 3, and other data standards. It can support high-wattage charging and power (see “How to tell whether a USB-C cable can carry high-wattage power and Thunderbolt 3 data.”)
It can be relatively easy to figure out if the port you have supports only what’s now called USB 3.2 Gen 2 (10 Gbps maximum)—also known as “SuperSpeed+” or “SuperSpeed USB 10Gbps”—or, in addition to the high-speed USB flavor, it also handles Thunderbolt 3.
All USB-C equipped Macs support Thunderbolt 3 except the now-discontinued 12-inch MacBook introduced in 2015. A few models of iPad include a USB-C connector that only supports the USB data protocols as well. (On non-Apple computers, you have to look up the specs.)
But how can you tell whether a cable with two USB-C ends will pass only USB up to 3.2 Gen 2 or up to the 40Gbps capable between two Thunderbolt 3-equipped devices, like a computer and a high-performance disk array?
Check the cable ends. A Thunderbolt 3 cable should have a lightning bolt symbol printed on the cable head on both ends. Some manufacturers may also stick a “3” on there, although the USB-C and Thunderbolt 2 cable shapes (the same as Mini DisplayPort) are distinctly different. A USB-only cable will show a SuperSpeed+ symbol, which may differ if it has high-wattage power suport.
If the cable is unlabeled, you can plug the cable in between a Thunderbolt 3-equipped Mac and a Thunderbolt 3 device. Does the Mac recognize the device? If not, it’s not a Thunderbolt 3 cable (or it’s defective).
If the Mac does recognize it, check that the hardware doesn’t automatically drop down to a compatible, slower standard than Thunderbolt 3 by using System Information (Option- > System Information), click Thunderbolt, and see if the device is listed there.
When it comes down to it, though, there’s an ipso facto situation, if I can drop some Latin: if, literally “on the face of it,” the cable lacks clear markings as to what standard it supports, I’d return or replace it, assuming the manufacturer didn’t take the care to label it properly, which would concern me about its production standards, too.
This Mac 911 article is in response to a question submitted by Macworld reader David.
Ask Mac 911
We’ve compiled a list of the questions we get asked most frequently along with answers and links to columns: read our super FAQ to see if your question is covered. If not, we’re always looking for new problems to solve! Email yours to firstname.lastname@example.org including screen captures as appropriate, and whether you want your full name used. Not every question will be answered, we don’t reply to email, and we cannot provide direct troubleshooting advice.