While running VirtualBox 4.1.4 on Mac OS X Lion as the host and Windows XP 3 as the guest, I was having some trouble getting my Western Digital Elements external hard drive recognized on the guest. The external was formatted for Windows and all my data was added from a Windows machine, so I could read it from the Mac, but I couldn't write to it. The Mac would recognize the external, but not the Windows guest. If I clicked on the USB icon at the bottom of the VirtualBox window, the external drive would be greyed out.
Oddly enough, if I started the VM first, had the Windows guest window selected, and THEN plugged in the external, XP would recognize it immediately. The host would not recognize it at all.
While the external was plugged into the Mac with my first attempts, I did add a filter for it under Ports > USB, but I'm not sure if that was really part of the solution.
Whatever. It's working!