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How Some Thunderbolt 3 Cables Underperform with USB-only Drives

You already know that the USB-C connection standard is difficult to sort out. As I wrote in “Explaining Thunderbolt 3, USB-C, and Everything In Between” (3 November 2016), USB-C is a hardware standard that allows peripheral controllers — a collection of firmware and chips and connectors — to pass various kinds of interface data. That includes USB 2.0, USB 3.0 and 3.1, Thunderbolt 2 and Thunderbolt 3, Ethernet, FireWire, DisplayPort, and others.

The same port on different computers and peripherals could have radically different capabilities. In the Apple world, look at the 12-inch MacBook, introduced in 2015, and both the 2016 and later MacBook Pro models and 2017 iMacs. The MacBook supports USB and DisplayPort natively; the newer Macs carry Thunderbolt 3, which the MacBook can’t handle. The Thunderbolt 3-equipped MacBook Pro and iMac models have a different, more capable controller.

Now there’s a new issue layered on top of that: AppleInsider uncovered a problem with certain kinds of cables designed for Thunderbolt 3 connections. They provided an intricate and technical explanation, which I confirmed through testing. I’d like to break that down into a form that’s easier to understand if you’re not a peripheral communications standards geek.

You can purchase four kinds of cables that have USB-C connectors on both ends:

  • USB-C power, which also includes USB 2.0 data transfer. These cables can vary from low wattage (typically up to 15 watts) to high wattage (as much as 100 watts), but carry data only at up to 480 Mbps. Apple uses a USB-C power cable for all its laptops to connect to the AC adapter. The data rate is secondary. I recommending avoiding low-wattage cables that only support USB 2.0 — they’re just about useless.

  • USB-C 3.1 Gen 1 and Gen 2. Despite the version number bump, USB 3.1 Gen 1 is the same as the original USB 3.0, 5 Gbps SuperSpeed standard, but with a USB-C plug instead of the familiar USB-A flat plug (with a blue separator inside) or the weirdly shaped 3.0 Micro B common on portable drives. It’s found on the MacBook. USB 3.1 Gen 2 (SuperSpeed+), which is 10 Gbps, is built into all Thunderbolt 3 Macs.

  • Thunderbolt 3 passive cables. These cables can carry the maximum 40 Gbps data rate between Thunderbolt 3 devices only at 18 inches (0.5 m) or less. At up to about 6.6 feet (2 m), passive cables support just 20 Gbps.

  • Thunderbolt 3 active cables. These more expensive cables provide 40 Gbps between Thunderbolt 3 devices at up to 6.6 feet (2 m) by incorporating circuitry into both ends of the cable. Some of these cables can also carry the maximum 100 watts of power allowed in the standard. You need this kind of cable for high-throughput Thunderbolt 3 peripherals, like SSD RAIDs used for video and animation.

The trouble that AppleInsider discovered arises only in a particular set of circumstances:

  • You have a hard drive with a USB-C port that supports only USB 3.1.
  • You’re using a Thunderbolt 3 cable to connect that drive to a Thunderbolt 3-capable computer.

  • The Thunderbolt 3 cable is active, rather than passive.

Instead of carrying up to 5 Gbps of USB 3.1 Gen 1 data, these active Thunderbolt cables throttle down to USB 2.0 speeds, offering about one-tenth as much throughput. With a passive Thunderbolt cable, USB 3.1 Gen 1 passes data as you’d expect.

I tested this with a G-Technology 1 TB USB-C (USB 3.1-only) hard disk drive, and was able to confirm AppleInsider’s results: about 35 megabytes per second (280 Mbps) of throughput with the active cable, and 130 megabytes per second (1 Gbps) of throughput with the passive one. That’s toward the upper range of what you would expect from that drive. With an SSD, the difference would be even more striking.

Thunderbolt 3 passive cables are far cheaper than the active ones. I paid $51 for a 2-meter active Thunderbolt 3 cable made by Cable Matters but just $25 for a 1-meter Pluggable Technologies passive Thunderbolt 3 cable. USB 3.1-only cables cost about half as much as passive Thunderbolt 3 cables.

It seems unlikely that you would intentionally buy an expensive active Thunderbolt 3 cable to use with a USB 3.1-only drive. In my experience, such drives ship with a USB 3.1-only cable! And USB 3.1 drives with USB-C connectors aren’t that common because there’s little advantage to using USB-C on both ends: a USB-3.0/3.1 Gen 1 cable with a USB-C jack on one end and an old style USB-A jack or newer Micro B on the other provides the same 5 Gbps maximum throughput.

However, I hope AppleInsider’s discovery (and my confirmation) will help explain mysterious performance problems you may have experienced with certain devices and cables. USB 3.1-only cables should be labeled with SS or SS+ on each end; Thunderbolt 3 cables have a lightning bolt with an arrow at its tapered end. That’s a subtle indication for what could be up to a tenfold difference in throughput.

When purchasing a Thunderbolt 3 cable, if you’re sure you want a passive one, it may not be labeled as such, but it will almost certainly be cheaper than an active cable of the same length, and, if longer than 1.5 feet (0.5 meters), labeled 20 Gbps. When sorting through a collection of Thunderbolt 3 cables you didn’t purchase, there’s nothing that will help you distinguish between active and passive cables, so I recommend labeling any cables you buy as soon as you receive them.

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