USB 3.2 Gen 2x2 disk not 20gigabit?

burkocan wrote:

Hi, I can get 920mb read and write speed on my Mac mini(m2) via Kingston XS2000 4TB ssd disk, but the theoretical speed should be 2000MB/sec that on average. Mac mini has thunderbolt 4 and usb 4 input and the theoretical speed is 40gigabit/sec. When I called the apple authorized service by phone, I got the answer that you are already using the fastest possible speed. How can I use this disk at the speed it should be? Do I need to use a new cable or any converter?

Thank you.

B

We know that the Mini and the SSD are going to fall back to USB 3.1 Gen 2 – "up to 10 Gigabits per second". 10 gigabits per second = 1,250 megabytes per second, so that's the maximum that you could get even if there was absolutely no overhead.

It is possible to get higher external SSD speeds – but for that, you'd need a Thunderbolt 3/4 SSD.

Based on Wikipedia – admittedly, not necessarily the most authoritative source – it appears that USB4 has three "up to 20 Gbps" USB transfer modes. The two new ones aren't compatible with each other or USB 3.2 Gen 2x2.

There is nothing prohibiting a particular USB4 host port from supporting all three. However,

• Support for one of the new USB4 "up to 20 Gbps" modes (the one with a marketing name) is mandatory for a USB4 host port.
• Support for USB 3.2 Gen 2x2 is optional for a USB4 host port.

It is my understanding that Macs do not implement USB 3.2 Gen 2x2. It may seem odd that Macs implement the optional USB4 "up to 40 Gbps" mode and optional Thunderbolt 3/4 "up to 40 Gbps" mode, but not the optional USB 3.2 Gen 2x2 one. Nonetheless, I believe that the standards allow this.

In general, USB 3.2 Gen 2x2 peripherals don't know about Thunderbolt or about the new USB4 "up to 20 Gbps" USB transfer modes. So when you plug them into a Mac, the two settle on traditional USB at a speed that both support. If you're talking about a USB-C port on an Apple Silicon Mac, that is typically USB 3.1 Gen 2 ("up to 10 Gbps") speed.

Directly from the MacMini Spec:

• Two Thunderbolt 4 ports (M2) or four Thunderbolt 4 ports (M2 Pro) with support for:
  • DisplayPort
  • Thunderbolt 4 (up to 40Gb/s)
  • USB 4 (up to 40Gb/s)
  • USB 3.1 Gen 2 (up to 10Gb/s)
  • Thunderbolt 2, HDMI, DVI, and VGA supported using adapters (sold separately)

So, if you want higher speeds, you either need to get a true USB4 SSD or a Thunderbolt 3/4 SSD (Thunderbolt 4 is basically just a refinement of the Thunderbolt 3 spec).

You're doing about as good as you're going to get with that drive, and it's pretty good under the circumstances.

The Mac Mini M2 has two USB interfaces ... USB 3.1 Gen 2 (10Gbps) and USB 4 (40Gbps) Since your Kingston drive is USB 3.2 Gen 2x2 -- not USB4 -- you are going to be limited to the speed of the Mini's USB 3.1 Gen2 (10Gbps) interface.

The key word here is “theoretical”. Actual performance can be affected by numerous variables, including cable length and quality, nearby electrical interference, etc. Marketing claims are based on laboratory conditions. It can also be affected by other devices using the same USB bus.

Further explanation ....

While USB is generally "backward compatible" with previous versions, that is no longer exactly true with USB4. USB4 uses a different data encoding protocol than all previous versions of USB. Your Mac Mini recognizes that your Kingston drive is not USB4 and therefore it switches (for backward compatibility) to its only available alternative, USB 3.2 (10Gbps). And, as always with USB, you get the lowest transfer speed available among the connected components. In your case, with this Mac Mini & this Kingston drive, that's 10Gbps.

also take a look at this Apple Support document...

About the Thunderbolt ports on Mac - Apple Support