

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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Comments about How Some Thunderbolt 3 Cables Underperform with USB-only Drives
(Comments are closed.)

Very interesting report. Thanks, Glenn.

I just have one issue. I think it might cause confusion to refer to "USB-C is a hardware standard... That includes ... Thunderbolt 2 and Thunderbolt 3...". The reason for this is that there are and will be many more USB-C devices/cables that are USB3-compatible, but don't "speak" TB. The reason for this is of course that USB is cheap compared to the higher-bandwidth TB and especially the PC world usually prefers cheap over performance.

My understanding is that Thunderbolt 3 always includes USB3, but USB3 does not necessarily mean TB3. So if somebody wants to connect a USB3 HDD to a TB3 port on a Mac that should always work. However, if you have a TB3 PCIe enclosure and try attaching that to a cheap PC's USB-C port chances are (at best) nothing will happen.

For this reason I think it's wise to always refer to TB3 devices/cables as TB and to use USB-C only when referring to using the USB protocol.

Although the two share the same physical connector they use different protocols. This is is made even more complicated by the fact that one protocol (TB3) can transport the other (USB3 Gen 1), but not the other way around. Exactly this last point makes it very dangerous to refer to TB3 as USB-C IMHO.
Glenn Fleishman  An apple icon for a TidBITS Staffer 2017-08-23 20:17
USB-C is the name of the port connection, and it's what it looks like, so it won’t reduce confusion to introduce a special nomenclature only we use.

I’m not clear that anyone will sell a Thunderbolt 3 enclosure or drive that doesn’t incorporate USB 3.1 Gen 1 or 2 as well. It’s possible, I guess, because USB isn't a requirement for Thunderbolt 3. Still, it seems unlikely. And it’s also weird that someone would wind up with a very expensive Thunderbolt 3 drive and plug it into a USB-only USB-C-equipped computer.

While you mention the PC world, we’re an Apple-focused publication, so we assume the majority of users are typically using a Mac, and will need more advice from other sources if they’re running Windows.
I believe that's exactly the problem. Here in the Mac world IMHO we should only use USB-C instead of TB3 when we mean USB-only. If we mean TB3 we IMHO shouldn't call it USB.

Otherwise we'll likely see lots of confused people when they go out to get TB3 devices/performance but decide to get the cheaper USB-C stuff only to then discover that you're not getting the same for less.
A bit less abstract, here's a recent issue.

Apple offers a TB-FW adapter converting TB2 from/to FW800. Apple also offers a TB3/TB2 dongle so that FW adapter can be connected to a TB3 port. A relative recently bought a MB and realized he could use his old FW800 drive together with his old FW-TB adapter if he just went out and bought a TB3/TB2 adapter. Because hey, he had USB-C and after all TB3 is just Apple's fancy name for what the entire rest of the industry calls USB-C, right? ;)

He was quite infuriated when he realized this setup wouldn't let him access his old FW800 drive and that he now had a useless TB3/TB2 adapter on his hands. I explained to him why it didn't work. Obviously it was his mistake since he was ill informed. But the incident I think illustrates why here in Mac land, it makes sense to educate people that TB3 and USB-C are really different despite having the same physical interface and that while TB3 offers all of USB-C, USB-C is not (and cannot replace) TB3.
Thank you, Glenn.

I use a MacBook with an Acer H277HU display (https://tidbits.com/article/16412). Since the Apple cable doesn't work with the display (which baffled and annoyed me), I assume that video is different from the data that this cable should support. Did I understand that correctly?

If I want a spare cable for my setup, should I be shopping for a USB 3.1 cable? Would a Thunderbolt passive cable likely work?

Thanks for any insight.
Glenn Fleishman  An apple icon for a TidBITS Staffer 2017-08-28 22:05
I don't know what you mean here by the Apple cable, sorry!
The USB-C cable for the Apple charger maybe?
As Simon guessed, it is the Apple cable for that came with the MacBook for charging the battery. Since it had a USB-C connector on each end, I assumed it would work with the Acer display and be higher quality than the cable that came with the Acer display. It didn't work, and (if I understood your article) now I know why.

My questions were related to obtaining a spare cable for the Acer display, which would provide audio and video from the MacBook to the display and power from the Acer display to the MacBook to charge the battery. (The cable that came with the Acer display does these things.) Would you expect a USB 3.1 cable to provide those capabilities? Would you expect a Thunderbolt passive cable to provide those capabilities?

Thanks for sharing your knowledge.
Glenn Fleishman  An apple icon for a TidBITS Staffer 2017-08-30 12:38
This is a fascinating question. Most power-focused cables omit USB 3.1 for cost. It's cheaper to make a USB 2.0 cable that carries high wattage over USB-C than it is to make a similar USB-C cable that also carries USB 3.1 Gen 1 or 2 data!

All USB-C cables have to carry at least 3 amps at 5 volts (15 watts), but some can be engineered to carry higher wattage using the Power Delivery 2.0 spec as well as USB 3.1. I haven't seen these in the wild, because they're usually paired with hardware, like the Acer. I haven't tested any such cables from third parties, though I'm sure they exist.
What about a TB3 cable? Would that ensure you get ~100 W power as well as USB data transfer at USB3 speeds?

Would it have to be an active cable or can you get that high wattage over TB3 cables even when they're passive?
Glenn Fleishman  An apple icon for a TidBITS Staffer 2017-08-30 13:43
No, Thunderbolt 3 cables can vary in wattage, too, although active cables seem like they mostly carry fairly high wattages. CalDigit has a great explanation (with marketing information about their cables, but lots of detail:


From what I can tell, passive cables can pass higher wattages, but you’ll need to read the specs on each cable to determine which work for your particular purposes. Some carry 100W, from what I can tell.
I've only been using apple since 1983- and USB for over a decade -but I find this article interesting on one level and near worthless on another. What is missing is some diagrams, jpg picvs or other help to figure out which way is up- and ewhat the blue divider in usb really represents. I'm running two minis, a macair, an 4g ipad and an ipad pro with everything from usb 2 to thunderbolt to usb3 connectors. As to speed- i re3cently got an OTGSSD 240GB to use for bacvkup- it has a blue usd on a 6 inch cord and an odd usb connector into the credit card sized unit. The odd connector is labeled SS usblogoB. It is fantstically fast. backing up about 80GB via time machine in a few minutes to start- and in about a minute when incremental mode is used.

Although I have a plethora of usb and or thunbderbolt cables of various diameters and lengths- the article is of little help soting out which
without a diagram- photo-the links help - but male and female micro b ?? are confused .
Adam Engst  An apple icon for a TidBITS Staffer 2017-08-29 09:24
You might follow the link in the article that goes to:


It's devilishly hard to find (or make) images of all the connectors that can be republished, and that page has a good selection of such images.
Brian S  2017-08-28 21:39
"...Apple products currently don’t offer USB 3.1 Gen 2 controllers."
So if I understand correctly ( doubtful, I'm not a hardware guy ), that statement includes my shiny new 2017 iMac which Apple says on its spec page "USB 3.1 Gen 2 (up to 10 Gbps)". Of course those are USB ports and not USB-C. It's entirely possible I've overlooked Glenn's explanation since that got my attention.
Glenn Fleishman  An apple icon for a TidBITS Staffer 2017-08-28 22:35
That’s my mistake—I even own a 2017 iMac! If you look at USB-C info on Apple’s site, it only discusses up to 3.1 Gen 1, but you’re right about the spec on the iMac! Good catch!

This has now been fixed in the article. Thanks!
Scott Randell  2017-08-29 13:57
Hi Glenn,
This is a very informative article. It seems however that introducing this all in one connector while may other protocols are still in place and actively been used is an unnecessary burden on consumers (us). I like the fact the the iMac has both Thunderbolt 3 and USB 3 connectors, and the Mac Mini has both Thunderbolt 2, and USB 3 connectors, and the Macbook Air also has Thunderbolt 2, and USB 3 connectors . This alleviates some of the confusion. The Macbook is a totally impractical product. Only one connector and it is only USB C. That means that you are relegated to USB peripherals and you will need adapters or docks for most of them. So you pay more for it than the Macbook Air and almost as much as the base model Macbook pro with the same specs (256 GB of storage and 16 GB of ram) For only $200.00 extra you get the Macbook pro which has more power and 2 TB3/USB C ports. Granted you may still have to buy some adapters for older peripherals but you still have a more versatile connector.

In addition, as someone else mentioned there is no way to use FW 800 or 400 peripherals with the Macbook. This having to be concerned about which cable to buy since there are (from what I can see) 3 different cables using the same (USB C) connector, all with different capabilities. I can’t speak for everyone, but I find this annoying.
You make a very good point about the MacBook, Scott.

It's very unfortunate Apple chose to leave it USB-C rather than just switch to a TB3 port. Maybe on the first gen MB it was understandable, but when they recently updated the MB they definitely should have switched to TB3. This leaves the MB in an odd situation in Mac land. It's the only recently updated Mac to not offer TB3. Stuck with non-TB3 USB-C it will - despite a plug that looks identical - not be able to work with many peripherals targeted at Mac users. Sure the PC world will offer plenty of cheap USB-C equipment, but much of that will not offer the same quality user-experience as TB3 hardware sold explicitly for use on a Mac with for example Mac software and Mac support.

That said, no doubt there is and will be good USB-C hardware. However, all of that would have worked just as well on a MacBook TB3 port, whereas now none of the good TB3 stuff works on a MacBook. Apple really dropped that ball.