I'm experiencing this problem as well. I'm running Unraid 6 as a guest on ESXI 5.1 (and a supermicro X9SCM motherboard with SASLP-MV8 raid controllers).
However, I'm unable to pass through the USB controller -- when I set the value to pass it through, it doesn't 'stick' through the reboot.
Any one know if an upgrade to the latest ESXI would correct this?
Edit: I've upgraded to 6.1rc and am still seeing this same issue:
Jul 18 23:42:22 Tower kernel: usb 1-1: reset high-speed USB device number 2 using ehci-pci
Jul 18 23:42:22 Tower kernel: sd 3:0:0:0: [sda] 121307136 512-byte logical blocks: (62.1 GB/57.8 GiB)