USB4 promises 40Gbps, but a BIOS switch made it USB 3 without telling me.

I was expecting my new USB4 enclosure to feel like a second internal drive. Instead, the Satechi USB4 NVMe SSD Pro clocked in at 1013MB/s, and for weeks I blamed the cable, then my chosen USB-C port, and then every other slot on the motherboard’s back panel.

The real culprit is a BIOS setting I disabled a few months ago. It turned my USB4 port into a plain old USB 3 port and Windows never said a word about it.

My USB4 case is stuck at USB 3 speed

First, cable took the blame

The setting is nothing exotic. My desktop runs an ASUS TUF Gaming X870-Plus WiFi and like every X870 board it has two USB4 ports on the back. Thanks to the NAND shortage, SSD prices have been rising all year, so I skipped buying a new portable drive. I removed the 2TB WD Black SN850X from my old unit and slid it into the Satechi case instead.

The first big backup was slow and the benchmark confirmed it. Sequential read speed 1013 MB/s, write speed 978 MB/s. A 48GB folder of RAW photos took 57 seconds to land on it. If you’ve used a regular 10Gbps USB drive, these numbers will look familiar because they’re where the USB 3.2 Gen 2 link is at the top.

So I did what everyone else did and blamed it on a short cable in the box. I bought a certified 40Gbps cable for $19 and got the same result, down to the rounding error. I’ve also tried a second USB4 port and the drive still checks out. I’m disappointed and a little betrayed because a 40Gb product that delivers 10Gb/s seems like a refund waiting to happen.

The device manager was responsive all the time

One word left him

The information is always sitting in the Device Manager. Under disk drives, my SN850X had SCSI Disk Device attached to its name, and it sat behind the USB Attached SCSI controller. This is how Windows lists a regular USB flash drive. A working USB4 enclosure exposes the SSD as a normal NVMe drive, just like on a motherboard.

This difference is due to one trick that USB4 has compared to the old USB. It can transport the SSD PCIe connection directly through a cable called PCIe tunneling. When tunneling off, the case reverts to USB 3.2 mode at 10Gbps, and you get exactly the same speeds I’ve seen.

That’s when I remembered the season. A pair of yellow warning icons appeared next to the PCI Express downstream switch port in Device Manager, and disabling the tunnel made them disappear. A subsequent BIOS update fixed these warnings, but I never reverted the setting and Windows never complained. Plot twist: The mistake was me.

It took a minute to reconnect the PCIe tunnel

And the drive finally woke up

On my ASUS board, the fix takes less than a minute:

1. Restart and click Delete Press to enter the BIOS F7 For advanced mode.

2. go through Advanced -> AMD PBS -> USB4 (ASM4242 controller).

3. Set PCIe Tunneling to Enabled.

4. Click the button F10 to save and reload.

MSI, Gigabyte and ASRock label it differently, so look for the word tunneling in the manual. Intel boards with Thunderbolt often refer to this as PCIE tunneling over USB4.

I reconnected the enclosure, and Device Manager listed the drive by its actual name, with no SCSI tag visible. Read speed increased to 3,618 MB/s, write speed increased to 3,274 MB/s. I laughed out loud at the performance of CrystalDiskMark, then ran it twice more in case the first one was a fluke. That 48GB photo folder was copied in 19 seconds.

That’s well within the range of Satechi’s promised 3,840MB/s. On paper, nothing comes close to the 4.8GB/s speed that 40Gbps does, According to ASUSbecause the PCIe tunnel inside ASMedia controller below the link itself. My actual copies are getting smaller as the folder fills up with smaller files, which is normal for any drive.

Windows has a quick way to confirm which mode you’re in. Open up Settings -> Bluetooth and devices -> USB -> USB4 hubs and devicesand you’ll see a list of connected USB4 devices and what they support. Microsoft says page only appears on computers that use their own USB4 connection manager, so you may not have it. If it is, check it before buying another cable.

There’s a good reason to turn it off

Tunneling allows devices to access your memory

For most desktops, it’s fine to leave the PCIe tunnel enabled, but it extends the capabilities of the connected device. A PCIe device can use direct memory access, so it can read and write system memory without first asking the CPU. This is great for SSDs, and even better if someone adds a malicious gadget to your computer while you’re away.

Thunderbolt laptops usually ask for your approval before connecting a new device. On ASUS AM5 boards, at least one owner has been found the only choices were to enable the tunnel or disable the tunnel without an approval step. Windows has a security layer called Kernel DMA Protection. Type msinfo32 into Start, then look for the DMA kernel protection path in the System Summary to see if new devices are blocked until you log on.

My desk never leaves my room, so I keep plugging the tunnel. If you bring a small computer to LAN parties, it’s not at all paranoid to turn it off for the weekend.

Check Device Manager before buying another cable

I spent weeks treating a 40Gb/s case like a cheap USB stick. The fix was a switch I turned off myself. If your USB4 drive is close to 1000MB/s, look for that SCSI tag in Device Manager before spending another dollar. As for that 1,013MB/s screenshot, it’s currently my desktop wallpaper as a reminder.