XPS17 + 3080ti on TB3 16Gbps? - Bad H2D  

After digging deep on the issue, I suspect that the cable cause this issue.

I now have three TB3 cable, 1 come with the Mantiz Saturn Pro, 1 Apple TB3 0.8m cable and 1 come with the 1080 Aorus Gaming Box. All those cables are passive cable and I suspect that may be the cause.

I have ordered 2 active cable and will report the result when they arrived.

@crossday, I hope this is an issue with the TB3 cables and not the host system firmware/configuration. It would be very strange for Dell to cripple the Thunderbolt connection after other mishaps when Thunderbolt 3 eGFX was first introduced.

it's getting wired. I've got a 1.5m TB3 active cable for the test. However, the fan goes crazy full speed once I plugged it and no egpu detected.Windows 10 popped this message "Thunderbolt device functionality might be limited". I now have no idea what's happening.

@itsage, Well I think I have figured it out.

I have been using the Apple 0.8m TB3 passive cable and plug it on the left port, and it only runs on 2-lane speed (USB3.2 Gen2?).

When the left port plugged in with a 1.5m active cable, it cannot defect the enclosure at all, i.e. the left slot can only use a 2-lane speed thunderbolt 3 (20Gbps bandwidth) with no support of an active cable.

When I switch to the right port, every thing comes back. No matter the passive or active cable gives full speed (40Gbps), which indicate the cables have nothing to do with this situation but Dell's problem.

What should be my next step? To seek help from Dell Support for a fix?

@crossday, Can you open HWiNFO64 as well as Device Manager and identify PCIe Bus numbers to the corresponding Thunderbolt 3 ports on the laptop? From the OP it seems the left TB3 ports get a 4-lane PCIe 3.0 connection, same as right TB3 ports. I wonder if they all are going through the PCH or have direct access to the CPU.

@itsage, unfortunately the miracle has gone after a reboot. Now my enclosure cannot be defected on ports on both side when using the active cable. Though the passive cable can still be used, but only 16Gbps.

@crossday, I would check with different BIOS versions before engaging Dell Support for an exchange.

@itsage, I managed to get the right ports working with full speed again after flashing bios. So far I've tested 3 BIOS version. Different BIOS version, same results: the left ports would never go full speed. And it seems that if I don't safely remove the enclosure before unplug it, it will show that functionality limited notification and would not connect anymore until I re-flash the BIOS (talking about right ports with active cable; active cable will never connect to PC and fan speed will instantly max out). If I can live with 2-lane, no matter how I manipulate the enclosure, it will still get recognize, just at 16Gbps speed.

 If I'm going to connect the egpu to external monitor, is it not much of a different between 2-lane and 4-lane?