I have the same problem with 2 x 4TB WD USB 3 external drives:
Bus 1 --> 0000:00:14.0 (IOMMU group 3)
Bus 001 Device 004: ID 046d:c248 Logitech, Inc. G105 Gaming Keyboard
Bus 001 Device 003: ID 8087:0a2b Intel Corp.
Bus 001 Device 002: ID 0781:5581 SanDisk Corp. Ultra
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 2 --> 0000:00:14.0 (IOMMU group 3)
Bus 002 Device 006: ID 1058:25e2 Western Digital Technologies, Inc. My Passport (WD40NMZW)
Bus 002 Device 005: ID 1058:25e2 Western Digital Technologies, Inc. My Passport (WD40NMZW)
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
I don't have any other USB controllers so cannot pass through the entire controller (micro ATX MB). Unfortunately I don't have the Linux chops to be able to make a coherent feature request. If I did it would be something along the lines of querying the usb device for unique Id parameters i.e.:
Then allowing selection of a unique identifying parameter like "serial" to make a selection, I don't really have another $125 to spend on a different model/brand drive so that's not really an option.