Speed Up Your Peripherals

USB and Thunderbolt are the most commonly used interfaces for the sorts of external devices (such as hard drives) that can most significantly affect your Mac’s speed. In this final chapter I get into some admittedly geeky territory, discussing somewhat obscure concepts like bus bandwidth—but it’s all in the service of speeding up your Mac. By understanding what I explain here, you can make informed decisions about which devices to connect to your Mac and how to do so in a way that keeps overall performance as high as possible.

Learn About Buses and Ports

If you look at your Mac, you’ll see one or more USB ports (physical connectors where you can attach cables) and perhaps one or more Thunderbolt ports. (There are other ports on some Macs, too, such as Ethernet and HDMI, but I’m ignoring those in this chapter.)

It’s fairly obvious that if you have three USB ports, you can plug in three USB devices. (And, if one of those devices is a hub, which increases the number of available ports, you can plug in still more devices.) All the ports of a given type look the same, but details about your Mac’s internal configuration could determine which of those ports you might want to attach a device to.

To make sense of this, you must understand the concept of a bus, which is essentially a particular set of physical circuits that carry some sort of data. Your Mac has many different buses for communicating with a variety of internal and external devices, and ordinarily you need never concern yourself with any of them. However, there’s a crucial fact about buses that can affect your Mac’s performance: every bus has a fixed amount of bandwidth, which is to say that no matter how many devices connect to it, only a certain amount of data can travel through the bus at any given time. So, if a bus has just one device on it, that device can use the bus’s full bandwidth; connect two or more, and the devices must share that bandwidth. Of course, all the devices may not actively be transferring data at any point in time, but to the extent that they do, the bus bandwidth can limit their effective speed.

Now here’s the interesting part: the number of USB or Thunderbolt buses your Mac has may be completely different from the number of ports it has! For example, your Mac may have only one USB port but two USB buses (the second of which is used for internal devices such as the trackpad and camera). Or it may have more USB ports than buses, meaning each bus controls multiple ports. If it’s a Mac Pro (2013), you have three Thunderbolt buses but six Thunderbolt ports—two for each bus (with the HDMI port sharing the third Thunderbolt bus).

Why should you care about this? It’s simple: knowing that each bus has a limited amount of bandwidth, you’ll get the best performance if you spread out the load. For example, let’s say you have two USB hard drives and two USB ports on your Mac, which you’ve discovered (as I explain in a moment) correspond to two different USB buses. You could connect one drive to each port, or you could connect a hub to one of the ports and connect both drives to the hub (leaving the other port free for additional devices). In this example, connecting one drive to each port is the better choice, because then the two drives don’t have to share the bandwidth of a single bus. If you need to attach more devices, you can still use a hub, plugging one of your drives (and the other devices) into it, but keeping that on a separate USB bus from the other drive will increase the performance of both.

So, how can you figure out how many buses of each type your Mac has, and which ports go with which buses? Follow these steps:

  1. Open System Information in /Applications/Utilities.

  2. In the list on the left, under Hardware, select USB (Figure 13).

    Figure 13: Select a USB device in the Device Tree to see its characteristics, including which bus it connects to.
    Figure 13: Select a USB device in the Device Tree to see its characteristics, including which bus it connects to.

  3. Look in the USB Device Tree on the right side of the window. You’ll see one or more entries at the top level for your Mac’s USB buses, listed by version: USB 2.0 Bus, USB 3.0 Bus, or USB 3.1 Bus. (Any of these may have “Hub” subentries—possibly nested more than one level deep—which represent internal or external USB hubs.) The characteristics of the device selected in the Device Tree are displayed below the tree.

  4. To figure out which port goes with which bus, take a device that you know for sure uses USB 2.0 or higher (such as a USB 3 hard drive) and plug it into one of your Mac’s USB ports. In a moment, it should show up in the Device Tree; if not, press ⌘‑R to refresh the display. Now make a note of which physical port you’ve connected the device to and which bus in the Device Tree the device appears on.

  5. Disconnect the USB device (remembering to eject it first if it’s a hard drive or flash drive). Then repeat step 4, using a different USB port this time. Again, note the results. Continue with all your Mac’s USB ports until you have a list of which physical ports correspond to which buses.

You can do the same sort of test with Thunderbolt ports and buses. Thunderbolt is less complicated in the sense that your Mac uses it only for external devices, not for internal devices as with USB. But it’s more complicated in the sense that there are three different versions of Thunderbolt—first-generation, Thunderbolt 2, and Thunderbolt 3—and System Information doesn’t use those terms; instead, you must look at the Speed entry for each port of a given Thunderbolt bus. First-generation Thunderbolt buses support only 10 Gbps ports, Thunderbolt 2 supports 20 Gbps ports, and Thunderbolt 3 supports up to 40 Mbps ports, though some Macs have Thunderbolt 3 buses in which one port supports 40 Mbps and the other supports only 20 Mbps.

I know, I know, it’s madness.

I can’t answer every Thunderbolt port-and-bus question in this book, but here are some quick facts about Thunderbolt buses on Macs:

  • All Macs with first-generation Thunderbolt ports or Thunderbolt 2 ports have a single Thunderbolt bus.

  • The Mac Pro (2013) has three Thunderbolt buses, and its bus-to-port mappings are illustrated in How to Configure 2013 Mac Pro Thunderbolt Connections for Maximum Performance.

  • The iMac (Retina 4K and Retina 5K, 2017) has two Thunderbolt 3 ports on a single Thunderbolt bus.

  • The MacBook Pro (15-inch, 2016 and 2017) has four Thunderbolt 3 ports on two Thunderbolt buses. Bus 0 is for the left-hand ports; Bus 1 is for the right-hand ports.

  • The MacBook Pro (13-inch, 2016 and 2017, Two Thunderbolt 3 Ports) has a single Thunderbolt bus, while the MacBook Pro (13-inch, 2016 and 2017, Four Thunderbolt 3 Ports) has two Thunderbolt buses. (If you have one, you can check its port-to-bus mapping using the instructions above.)

    However, there’s an oddity specific to a single model—the MacBook Pro (13-inch, 2016, Four Thunderbolt 3 Ports): only the two left-hand ports support full Thunderbolt 3 performance; the right-hand ports have reduced throughput.

  • The iMac Pro (2017) has four Thunderbolt 3 ports on two Thunderbolt buses. (Again, if you have one, you can check its port-to-bus mapping using the instructions above.)

Explore USB and Thunderbolt Options

Now that you know which of your USB and Thunderbolt ports use which buses, you can make smarter decisions about which port to use in order to get the best performance. All things being equal, try to spread out the load; if feasible, you should attach your highest-speed peripherals to ports that use different buses instead of the same bus.

So far so good, but there’s more to the story. I’ve mentioned in passing that USB and Thunderbolt interfaces each come in several varieties, and though they’re generally both forward- and backward-compatible, you’ll want to make sure you choose the best (that is, fastest) available interface compatible with your Mac and your budget. But which kind of USB or Thunderbolt you can or should use in a given situation is a tricky question, so I’d like to spell out the details here as best I can.

USB Varieties

USB has gone through several major revisions and, for the most part, newer versions (with higher numbers) are faster than older ones:

  • USB 1.0 (Low Speed): The original version of USB, which you’ll rarely encounter anymore, offered a data transfer rate of only 1.5 Mbps.

  • USB 1.1 (Full Speed): With a 12 Mbps data rate, USB 1.1 is fine for peripherals that transfer only small amounts of data, such as keyboards and mice.

  • USB 2.0 (High Speed): Many Macs and peripherals, including external hard drives, supported the 480 Mbps USB 2.0 standard. If you can avoid it, do; although the speed was great 10 years ago, it’s painful today.

  • USB 3.0 (Super Speed): Most Mac models released in mid-2012 or later have 5 Gbps USB 3.0 ports. Likewise, most currently available external hard drives with USB connections use USB 3.0.

  • USB 3.1 Gen 1 (Super Speed): USB 3.1 is the first version of USB to use the new, smaller, and (finally!) reversible Type-C connector. (USB 3.1 ports and connectors are thus often referred to as “USB-C.”) However, the connector is the only thing that makes USB 3.1 Gen 1 different from USB 3.0; it still transfers data at 5 Gbps.

  • USB 3.1 Gen 2 (Super Speed+): Using the same USB-C connector as USB 3.1 Gen 1, Gen 2 doubles the data transfer rate to 10 Gbps. All Macs that support USB 3.1 Gen 2 also support Thunderbolt 3, which uses the same connector (see Thunderbolt Varieties, ahead). But most hard drives and other peripherals that use USB 3.1 Gen 2 do not also use Thunderbolt 3.

  • USB 3.2, Gen 3 (Super Speed+): This brand-new standard (as of September 2017) supports 20 Gbps—twice the speed of Gen 2. I expect it will be a while before it shows up on new Macs.

All the USB versions from 1.0 through 3.0 use exactly the same connectors, and are all interoperable—but the slower speed always wins. A USB 2.0 drive connected to a USB 3.0 port still runs at USB 2.0 speeds, as does a USB 3.0 drive connected to a USB 3.0 port. So, given the choice, you’ll always want to use a device that offers the highest possible speed that your Mac also supports.

But here’s the problem: you can’t tell just by looking at a device or a plug which version of USB it supports, and therefore how fast it will be. (Some manufacturers thoughtfully put initials like HS or SS—indicating High Speed or Super Speed—on or near their connectors, but that’s entirely optional.) If the device’s speed isn’t stated on a label, you might be able to look it up on the web. Otherwise, the best way to tell for sure is to plug it into your Mac, select it in System Information (as described in Learn About Buses and Ports), and see what’s listed in the “Speed” row—for example “Up to 12 Mb/sec” means it’s USB 1.1.

Surely that problem went away with USB 3.1, though, right? Yeah, not so much.

All USB 3.1 plugs and ports use the same USB-C connector. So, once again, you can’t tell which speed a given device or cable supports just by looking at it, unless it happens to be labeled in some way.

In fact, it’s even worse than that:

  • A cable can have USB-C connectors on both ends but still only transfer data at the USB 2.0 rate of 480 Mbps! This is the case with the USB-C charging cables that Apple ships with the MacBook Pro (2016 and later). If you want USB 3.0 (let alone USB 3.1 Gen 2) speeds, you’ll need to buy a cable designed to support them.

  • USB-C cables look exactly like Thunderbolt 3 cables, but they don’t act the same. If you want the full 40 Gbps throughput of Thunderbolt 3, you must use a cable specifically designed for that purpose. And, paradoxically, the very best Thunderbolt cable money can buy can, if used to attach a USB 3.1 Gen 1 hard drive to your Mac, result in achingly slow, 480 Mbps, USB 2.0 speeds. (See USB-C and Thunderbolt Cables, ahead.)

To summarize thus far: to get the maximum speed out of any external peripherals connected to your Mac with USB, you’ll want to be sure those peripherals, and the cable you use, support the fastest version of USB your Mac offers. That may require research on the web, or testing in System Information, to get the complete story.

Thunderbolt Varieties

The story of Thunderbolt is shorter and less complicated than that of USB. But again, there are multiple versions, which get faster over time:

  • Thunderbolt: Thunderbolt version 1, or what I refer to as first-generation Thunderbolt, has two separate channels, each operating at up to 10 Mbps (the same speed as USB 3.1 Gen 2). It uses the Mini DisplayPort connector, and also supports the DisplayPort protocol (meaning you can connect a display to the Thunderbolt port on a Mac and it will work).

  • Thunderbolt 2: The second-generation Thunderbolt makes it possible for devices to combine the two 10 Mbps channels for 20 Mbps of throughput; it also supports a newer version (1.2) of the DisplayPort protocol, while using the same connector as before.

  • Thunderbolt 3: The latest version of Thunderbolt offers a maximum data transfer rate of 40 Mbps, and uses the USB-C connector (rather than Mini DisplayPort). It still supports the DisplayPort protocol, as well as USB 3.1 Gen 2, HDMI, audio, Ethernet, USB power delivery, and more. In other words: given the right adapter, you can connect a huge variety of devices to a Thunderbolt 3 port.

The first two versions of Thunderbolt caused a bit of confusion due to their use of the Mini DisplayPort connector; although you could connect a display to a Mac’s Thunderbolt port, attaching a Thunderbolt hard drive to a PC’s Mini DisplayPort wouldn’t work even though the cable and connectors are the same.

With Thunderbolt 3, you have the same problem yet again, in a slightly different form. Some Macs—notably, the MacBook (Retina, 2015–2017)—have USB-C ports that support both USB and DisplayPort, but because they aren’t Thunderbolt 3 ports, you can’t use them with Thunderbolt peripherals. Macs that do have Thunderbolt 3 ports—and can thus connect to any peripheral that has a USB-C connector—include the MacBook Pro (2016 and later), the MacBook Air (Retina, 2018), and the iMac Pro.

Once again, given the choice, you’ll want to use the peripherals that support the fastest version of Thunderbolt your Mac offers. You can connect a Thunderbolt or Thunderbolt 2 peripheral to a Thunderbolt 3 port (or vice versa) using an adapter, but you’ll be limited to the speed of the slower device.

Even that isn’t the end of the story, however! Not all Thunderbolt (or USB-C) cables are created equal, even though they may look the same. You’ll need the right one for best performance, as I discuss next.

USB-C and Thunderbolt Cables

If you have a Thunderbolt (version 1 or 2) device, and a Mac with a Thunderbolt (version 1 or 2) port, any Thunderbolt cable with the right connectors should work. (You’ll pay far more for optical Thunderbolt cables, which can be much longer than conventional copper cables.)

But when it comes to USB-C cables, used by both USB 3.1 and Thunderbolt 3, things get murky. As Glenn Fleishman explains in How Some Thunderbolt 3 Cables Underperform with USB-only Drives, there are four different kinds of cables with USB-C connectors, and it can be quite challenging to tell what kind of cable you have just by looking at it. (Your best bet is to study the manufacturer’s specs before buying the cable, and then label it so you’ll know in the future.) To summarize Glenn’s findings, your options are:

  • USB-C power: These cables support power delivery but transfer data at only 480 Mbps, like USB 2.0.

  • USB 3.1: These cables support up to the 10 Mbps of USB 3.1 Gen 2, as well as power delivery.

  • Thunderbolt 3, passive: Short cables (up to about 0.5m) support the full 40 Mbps data rate; longer ones (up to about 2m) max out at 20 Mbps.

  • Thunderbolt 3, active: These specially expensive cables support the 40 Mbps data rate at distances up to 2m, and also, in some cases, power delivery.

Crucially, if you use an active Thunderbolt 3 cable to connect a USB 3.x hard drive to a Mac, you’ll get a maximum data transfer rate of only 480 Mbps, even though you have a great cable connected to a super-fast port. The reason, in a nutshell, is that active Thunderbolt 3 cables don’t support USB 3.0 or 3.1. I’m sure that limitation made sense to the engineers who designed these things, but…geez. (Moral of the story: Use a USB 3.1 cable, or a passive Thunderbolt 3 cable, to connect a USB hard drive to your Mac. Use an active Thunderbolt 3 cable only for peripherals that explicitly support Thunderbolt 3.)

USB and Thunderbolt Hubs and Docks

After all that talk about ports, buses, protocol versions, and cables, there’s still one more topic to know about that can affect the speed of communication between your Mac and its peripherals. If your Mac doesn’t have enough ports, or the right kinds of ports, or ports with the right kinds of connectors located in the right places, you can attach a hub or dock. These gadgets are fantastically useful, but you need to know what you’re getting yourself into.

USB Hubs

If you attach a USB hub to your Mac (to get more USB ports), then no matter what you connect to that hub, its throughput will be limited to the top speed the hub supports. So, if your Mac has a USB 3 port but you connect a USB 2 hub to it, then everything attached to the hub will be limited to USB 2 speed. In other words: make sure the speed of your USB hub is at least as high as that of your Mac’s USB port.

But there’s another qualification. If two or more USB devices connected to a hub are actively transferring data at the same time, they may slow each other down if the total bandwidth of the devices exceeds the bandwidth of the USB bus. (That is, adding devices to a hub doesn’t magically multiply the bandwidth of the bus it’s connected to.) Therefore, if you have two or more USB devices and you want them to transfer data simultaneously at the maximum possible speed, attach them—directly or via hubs—to ports on your Mac belonging to different USB buses (where possible), as described in the previous section.

USB-C and Thunderbolt Docks

Unlike a hub, which simply multiplies ports, a dock gives you a greater variety of ports. There are USB-C docks that include various combinations of USB 3.1, USB 3.0, Ethernet, audio, and video ports, as well as memory card readers and pass-through power delivery (for charging your Mac laptop through the same cable that connects to the dock). And there are Thunderbolt 3 docks including any or all of the above—often in greater quantities or with faster throughput—plus FireWire, eSATA, S/PDIF digital audio out, and perhaps others.

As with USB hubs, bear in mind that a dock can multiply ports but not bandwidth. If you’re connecting to a USB 3.1 Gen 1 port, everything that attaches to your dock has to share that port’s 5 Gbps bandwidth. And even though a Thunderbolt 3 dock can, in principle, share 40 Gbps of bandwidth, limitations in the current generation of controller chips and other engineering considerations may mean you can’t get as much speed from each of your dock’s ports as you might expect.

I’ve spent many hours studying and comparing USB-C and Thunderbolt docks. I’ve concluded that the perfect dock for my own needs doesn’t yet exist (I know, first-world problems), but the reason is nearly always a single thing. For example, Dock A would be perfect if it supported 87w power delivery instead of just 60w, Dock B has that power delivery but needs one more USB-C port, Dock C has that extra port but it’s limited to USB 3.0 speeds, or whatever. So, to summarize: read the fine print. You may be able to connect a dock to your Mac, hook up all your devices, and get them to work, but if you didn’t notice that the port you want to use is way slower than the device you connect to it, c’est dommage.

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