This appears to be a VB bug not confined to host rf guest as per VB bug tracker #7462.
Quite illogical and hard to figure out. So don't use more than one virtual processor if you need USB devices to work.
USB devices not recognised if more than one processor chosen
USB devices not recognised if more than one processor chosen
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MBP 17" early 2011 2.3GHz; MacOS 10.6.8 Server (64bit); Windows7 (64bit); VB 4.2.18; BootCamp 3.3
MBP 17" early 2011 2.3GHz; MacOS 10.6.8 Server (64bit); Windows7 (64bit); VB 4.2.18; BootCamp 3.3