On my Dell 7010 and running ESXI client to connect to ESXI 5.5 host. I have a guest running and can't connect a USB flash drive when the quest is powered on. I install a USB 3.0 express card by Renesas Electronics USB host adapter and load the windows driver. Windows can see a USB flash drive with no problem just not ESXI quest or any of my other USB ports, from the card or mother board etc. But on a different Dell OptiPlex machine this works ok. Same version windows 7 and same client vSphere client version. Just different Dell machine 780 vs 7010.