I'm using Virtualbox 4.3.6 and having the same issue since forever. My host is Windows 7 SP1 32 bit, same for guest. I have Add-Ons installed and guest has Guest additions installed.
I plug in USB device (e.g. flash drive) only after guest is fully booted. It is visible in the host but not gets auto grabbed bu guest. If i try to manually attach it to the guest, the USB drive blinks and it's status changes from busy to captured, but nothing else happens. It is still visible in the host but not in the guest. If i try to attach it to the guest a second time i get an error that device is busy with previous request.
I have tried plugging the device to all 4 USB ports on my machine, tried rebooting both host and guest, tried adding an empty filter in USB properties of the guest. Nothing helps.
Boy, one would think that the devs could find some time to fix this issue since 2009 instead of dragging from one release to another