If you’re confused by the increasingly complicated world of USB-C, USB 3.2 Gen 2×2, Thunderbolt 3 and Thunderbolt 4, here we explain the difference in speeds and standards.
While Thunderbolt is relatively easy to understand because it was custom designed for Apple devices, USB-C with its various 3.0, 3.1, 3.2 and 3.2 Generation 2, 3.2 Generation 2×2 standards and the speed you get from them when connected to a Mac is complicated to understand.
This confusion has led to particular disappointment among Apple Silicon M1 Mac users who, even with the M1 Pro, M1 Max and M1 Ultra chips, have bought external hard drives promising incredibly fast data transfer speeds only to achieve less than half of what they claim to be capable of in reality.
Here we try to explain why this is happening so you can make a better choice when buying and external hard drive or external display for your Mac.
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Bytes & Bits
Firstly, it’s important to understand the difference between Bytes and Bits which is how data transfer speeds are measured.
1 byte = 8 bits.
So 1 Gigabyte (GB) = 1000 Megabit (MB) = 8 Gigabits (Gb).
Most external drives advertise their maximum data transfer speeds in Megabits per second (MB/s).
External drives have both a read speed (the time it takes to read data) and a write speed (the time it takes to write the data on the disk) which are usually similar but write speeds are often a bit slower.
So the SanDisk Extreme Pro SSD drive for example claims to offer transfer speeds of 2000MB/s via a USB-C standard known as USB 3.2 2×2 which equals 16Gb/s.
However, Mac users will get less than half these speeds with USB 3.2 2×2 connections as we’re about to explain.
What Is Thunderbolt?
Thunderbolt is a technology first developed between Apple and Intel back in 2011 which supports incredibly high data transfer speeds via a Thunderbolt port and a Thunderbolt cable.
Thunderbolt can support speeds of up to 40Gb/s (5000MB/s) which only USB 4.0 can compare with (more on this later) although you won’t find any external hard drives or monitors that can utilize all of this capacity due to physical performance limitations of the drives.
Since around 2107, all new Macs have a Thunderbolt 3 port and the latest M1 MacBook Pro and Mac Studio have Thunderbolt 4 ports.
The maximum speed of Thunderbolt 3 and 4 is exactly the same but Thunderbolt 4 offers slightly more flexibility and power when connecting multiple monitors to a Mac.
For example Thunderbolt 3 is only required to support one external 4K monitor whereas Thunderbolt 4 has to support at least two 4K displays or one 8K display.
Some longer Thunderbolt 3 cables are also limited to only 20GB/s whereas Thunderbolt 4 cables up to 2 meters support the maximum speed of 40Gb/s.
What Is USB-C?
USB-C is a wider industry standard for connecting USB-C supported devices to both PC and Macs.
USB-C ports are exactly the same oval shape as Thunderbolt ports so Mac users can use USB-C external drives, monitors and other devices.
However, the confusion starts when trying to understand the data transfer speeds you get with USB-C on a Mac (see more on this in USB-C designations below).
Because USB-C was not specifically designed for Macs like Thunderbolt is, the speeds that many PC users will get is different to the speeds Mac users will get.
Within USB-C there are now many different “designations” which offer different speeds which is where the confusion really starts for Mac (and many PC) users.
Here’s a comparison of how USB-C designations compare with each other, USB 4.0 and Thunderbolt:
- 5 Gb/s – USB 3.0, USB 3.1 Gen 1, USB 3.2 Gen 1
- 10 Gb/s – USB 3.1 Gen 2, USB 3.2 Gen 2
- 20 Gb/s – USB 3.2 Gen 2×2
- 40 Gb/s – USB 4.0
- 40 Gb/s – Thunderbolt (3 and 4)
USB 3.2 Gen 2×2
Many external drives such as the SanDIsk Extreme Pro now support USB 3.2 Gen 2×2 which is basically an ultra fast USB 3.2 connection with dual lanes.
USB 3.2 2×2 supports 2 lanes of data going at 10Gb/s both ways to reach the maximum speed of 20Gb/s.
Macs however (and still many PCs) can only support one lane of data flow through USB 3.2 2×2 cables and devices though and so you will only get USB 3.1 Gen 2 speeds on a Mac i.e. 10Gb/s via one single lane.
Any USB-C external drive that supports USB 3-2 2×2 will therefore claim to offer up to 2000 MB/s read/write speed but that’s only true if your computer supports it.
Not even Thunderbolt 4 in the latest M1 Macs will support the maximum speed of USB 3.2 2×2 standard because the technologies used in USB 3.2 2×2 and Thunderbolt are slightly different.
Therefore Mac users using the SanDisk Extreme Pro SSD drive on any Mac including M1 Macs will only see speeds of less than half the advertised transfer speed of 2000 MB/s which means around 1000 MB/s.
This is still pretty fast however and means you can transfer an 85GB file to a USB 3.2 2×2 external drive in just under 2 minutes and transfer it back to your Mac in around the same time.
In reality though, the read/write speeds will be even slower than this due to the way the hard drive throttles speeds to prevent over heating and the size of something known as the SLC cache.
It’s therefore possible that you’ll see transfer speeds of less than 1000 MB/s on an M1 Mac with a USB 3.2 2×2 external hard drive like those in testing below.
USB4 is the next evolution of USB-C ports and has the same oval shaped port that you can connect other USB-C and Thunderbolt devices to.
Currently however, many USB-C supporting devices won’t even work with a USB 4.0 port and it’s up to the manufacturer to make their devices backward compatible.
USB 4.0 is capable of the same 40Gb/s speeds as Thunderbolt 4 but as yet, there are very few USB4 devices available and USB 3.2 is still by far the dominant port used in external drives and other devices.
USB 4 vs Thunderbolt
USB 4 and Thunderbolt are the fastest data transfer standards and can achieve speeds of up to 40Gb/s.
They do not use the same technology though so you won’t be able to get 40Gb/s with a USB 4.0 device connected to a Mac.
This is because you can only get 40Gb/s with Thunderbolt on a Mac because Thunderbolt ports require an Intel controller in the external hard drive while USB does not.
So as with USB 3.0, 3.1 and 3.2, you can still connect USB 4.0 devices to a Mac but you just won’t enjoy the maximum transfer speed that it offers.
However, confusingly not all USB 3.0, 3.1, 3.2 or 3.2 2×2 standard supporting devices will work with USB 4.0.
It is entirely up to the manufacturer to make their USB 4.0 external drives or devices compatible with older USB-C standards and in some cases, they choose not to.
Note that the latest M1 Macs such as the MacBook Pro and Mac Studio have Thunderbolt 4 ports which are advertised as “supporting USB 4.0” but as explained already, you won’t be able to get USB 4.0 speeds with them.
Thunderbolt 4 (TB4) requires an Intel controller inside to reach maximum speed and so a USB 3.2 Gen 2×2 external drive will not achieve maximum speeds with a Thunderbolt 4 port.
Like USB 3.2 2×2, TB4 also uses dual lane data transmission to achieve 40Gb/s (20Gb/s in each lane) but only with an Intel controller inside which USB 3.2 Gen 2×2 drives cannot fully utilize.
Thunderbolt 4 is superior to USB 3.2 Gen 2×2 though because it also allows you to connect external displays, daisy chain multiple drives, and connect docks to them.
Thunderbolt devices can also charge your MacBook while connected to it by providing up to 100 watts of charging whereas USB 3.2 devices can’t.
USB 4.0 can support many of the same things as Thunderbolt 4 but it is entirely optional on the part of the manufacturer.
The top and bottom of this confusing situation is the only way to guarantee you get the fastest data transfer speeds in an external hard drive or the maximum spec support for an external monitor is to get a dedicated thunderbolt external hard drive for your Mac.
Any external hard drive that claims to offer speeds of up to 2000MB/s via a USB 3.2 2×2 will only achieve less than half of this in reality on a Mac.
If you’re a professional video editor, this still might still be fast enough if you just need to edit 4 or 5 streams of 4K footage directly from the drive.
Bu if you need to edit multiple streams of 6K and 8K video or need external monitor support, then you’re much better off getting a Thunderbolt external drive.