LimeSDR external power fries computer USB

I have been using my LimeSDR for many months on USB with no problem only as a receiver. I had previously tried using an external power source along with the USB only to find, when connecting the LimeSDR to USB, the PC shut down and the PC USB port was fried. After emailing the support line they said they could not help and I should find a local repair shop to look at the PCB.

I recently read a post about shorting R167 to allow FPF3042 to be by-passed. This allowed me to use the LimeSDR again but only with USB power. I tried some transmit tests yesterday (using SDR Angel) only to find it send out a short pulse then stop. After some further reading I saw suggestions about using external power, rather than USB power. I removed the short on R167 and applied a short to R166 instead.
After adding 12V power from a stable and accurate linear power supply the LimeSDR lit up as expected. I reattached the LimeSDR to the computer USB and then applied the power. The PC immediately shut down and the 4-way USB adapter I connected it to appears to have let out the magic smoke - another dead USB device!

The LimeSDR appears to be still powering up when connected by 12V - I just cannot connect my PC to it, which means it is an expensive red/green flashing lamp.

I am assuming (not always the best thing to do) that the 12V VCC_EXT is being sent down the VCC5P0_USB line to the USB port from the FPF3042 which appears to have failed. I see that FPF3042 are now obsolete, so I cannot swap it out.

Can anyone suggest how I can avoid blowing another USB port up and use the LimeSDR with both the external power source and connectivity with my PC?

I’ve just checked the voltages on VCC_EXT and VCC_INT at the points between the USB port and the DC input. Both show the same voltage when power is applied on the DC input jack. I tested with 6V-9V.

Would removing FPF3042 and keeping the jumper on R166 solve my problem and allow me to power the device from DC but also use USB data? In fact, do I really need to power from DC at all, should USB provide enough power for reliable transmit?

Tagging @zack.