Thunderbolt 3 Pro Cable USB 4 compatibility

[Dr McKay]

I have an Apple Thunderbolt 3 Pro cable (ML8E3ZM/A) which I have plugged into a Lenovo Thunderbolt 3 dock connecting a laptop via its USB 4 port.

Most Thunderbolt 3 cables are supposed to support 40Gbps on USB4 but this apparently isn’t a guarantee, Apples Thunderbolt 4 and 5 Pro cables list this but I’m unable to find their old store page for the Thunderbolt 3 Pro cable.

My question is, can the Thunderbolt 3 Pro cable support 40Gbps over usb 4 or is it limited to USB 3.1’s 10Gbps? I only ask because I have an external USB which could in theory max out the 10Gbps bandwidth and I don’t want to limit it.

 

[PaulD-UK]

"...connecting a laptop via its USB 4 port."

Apple Silicon Macs have TB4 ports, which also support USB4 because it uses the same protocol.
So it's not clear if you are referring to some other sort of laptop?
If it's not a Mac then the full TB3 spec is optional in the USB-IF's USB4 specs, so may be supported or not...

For cables over 0.8m the TB3 spec allows cables the option to only support 20Gbps, although 40Gbps is the norm.

But Apple's TB3 cables were produced to support the 2019 Mac Pro run the Pro XDR Display, so all Apple branded cables are fully compatible with the 40Gbps maximum bandwidth.

I don't have experience of USB4 SSDs, but if they are plugged into a TB3 dock, then they can't operate as USB4, and should revert to TB3 at full speed.*

"I have an external USB..."
Without knowing if you are talking about an enclosure/device or a monitor it should operate at TB3 40Gbps protocol speeds.

*But there are some USB4 devices, like OWC's Express 1M2 USB4 SSD enclosure, that don't support TB3, and instead drop down to USB 3.1’s 10Gbps if USB4 isn't available.