USB4 promises 40Gbps speeds, but one BIOS toggle quietly turned it into USB 3 without telling me

I expected my new USB4 enclosure to feel like a second internal drive. Instead, the Satechi USB4 NVMe SSD Pro crawled along at 1,013 MB/s, and for weeks I blamed the cable, then the USB-C port I'd picked, then every other jack on my motherboard's rear panel.

The real culprit was a BIOS setting I'd switched off months earlier. It quietly turned my USB4 port into plain old USB 3, and Windows never said a word about it.

My USB4 enclosure was stuck at USB 3 speeds

The cable took the blame first

The setup 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. SSD prices have kept climbing all year thanks to the NAND shortage, so I skipped buying a new portable drive. I pulled the 2TB WD Black SN850X out of my old build and slid it into the Satechi enclosure instead.

The first big backup felt slow, and the benchmark confirmed it. Sequential reads sat at 1,013 MB/s and writes at 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, those numbers look familiar, because they're right where a USB 3.2 Gen 2 link tops out.

So I did what everyone does and blamed the short cable in the box. I bought a certified 40Gbps cable for $19 and got the same result, down to a rounding error. I tried the second USB4 port too, and the drive still crawled. I was annoyed and a little betrayed, because a 40Gbps product delivering 10Gbps speeds feels like a refund waiting to happen.

Device Manager had the answer the whole time

One word gave it away

The clue was sitting in Device Manager the whole time. Under Disk drives, my SN850X had SCSI Disk Device tacked onto its name, and it sat behind a USB Attached SCSI controller. That's how Windows lists an ordinary USB stick. A working USB4 enclosure shows the SSD as a plain NVMe drive, just like it would look on the motherboard.

That difference comes down to one trick USB4 has over older USB. It can carry the SSD's PCIe connection straight through the cable, called PCIe tunneling. With tunneling off, the enclosure falls back to USB 3.2 mode at 10Gbps, and you get exactly the speeds I was seeing.

Then I remembered June. A pair of yellow warning icons had appeared next to PCI Express Downstream Switch Port in Device Manager, and turning tunneling off made them disappear. A later BIOS update fixed those warnings, but I never flipped the setting back, and Windows never complained. Plot twist: the bug was me.

Turning PCIe tunneling back on took a minute

And the drive finally woke up

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

1. Restart and press Delete to enter the BIOS, then press F7 for Advanced Mode.
2. Go to Advanced -> AMD PBS -> USB4 (ASM4242 Controller).
3. Set PCIe Tunneling to Enabled.
4. Press F10 to save and reboot.

MSI, Gigabyte, and ASRock label it a little differently, so search your manual for the word tunneling. Intel boards with Thunderbolt often call it PCIE Tunneling over USB4.

I plugged the enclosure back in, and Device Manager listed the drive by its real name, with no SCSI tag in sight. Reads jumped to 3,618 MB/s and writes to 3,274 MB/s. I laughed out loud at that CrystalDiskMark run, then ran it twice more in case the first one was a fluke. The same 48GB photo folder copied in 19 seconds.

That's within shouting distance of the 3,840 MB/s Satechi promises. Nothing gets near the 4.8 GB/s that 40Gbps works out to on paper, according to ASUS, because the PCIe tunnel inside the ASMedia controller tops out lower than the link itself. My real copies also dipped whenever a folder was packed with tiny files, which is normal for any drive.

Windows also has a quick way to confirm which mode you're in. Open Settings -> Bluetooth & devices -> USB -> USB4 Hubs and Devices, and you'll see a tree of connected USB4 devices and what they support. Microsoft says the page only appears on PCs using its own USB4 connection manager, so it may be missing on yours. If it's there, check it before you buy another cable.

There's a good reason it can be switched off

Tunneling lets devices reach your memory

For most desktops, leaving PCIe tunneling on is fine, but it does widen what a plugged-in device can reach. A PCIe device can use direct memory access, so it can read and write system memory without asking the CPU first. That's great for an SSD and much less great if someone plugs a malicious gadget into your PC while you're away.

Thunderbolt laptops usually ask you to approve a new device before it connects. On ASUS AM5 boards, at least one owner found the only choice was to tunnel on with no approval step, or tunnel off. Windows does have a safety net called Kernel DMA Protection. Type msinfo32 into Start, then look for the Kernel DMA Protection line in System Summary to see whether new devices are fenced off until you sign in.

My desktop never leaves my room, so I'm keeping tunneling on. If you haul a small PC to LAN parties, switching it off for the weekend isn't paranoid at all.

Check Device Manager before you buy another cable

I spent weeks treating a 40Gbps enclosure like a cheap USB stick. The fix was a toggle I'd turned off myself. If your USB4 drive is parked near 1,000 MB/s, look for that SCSI tag in Device Manager before you spend another dollar. As for that 1,013 MB/s screenshot, it's my desktop wallpaper now, as a warning.