for I've been asked by trac@VB.o to publish this here:
VirtualBox for OS-X seems to have the same permission problems described for Linux.
If I connect an USB drive, it appears as "unavailable" in VBoxManage or grayed-out in then guest-GUI dropdown menue, no matter what filter I define in the host settings.
I found this beeing a permission problem. For a quick test, start VirtualBox.app as root.Update 2009-11-26: The permission problem seems to be superseded. Search below for IOUSBMassStorageClass.
If your USB device is not longer grayed-out, you may have the same problem.
BTW - this does not occur using an USB-Stick. Not tested with other devices. I guess, network devices (/dev/vboxnetctl) and disks (/dev/vboxdrv) are affected, other USB devices not.
Problem could be fixed by adding write-permissions to /dev/vboxdrv in the Host.
For details, see http://te.tl/vg
This appeared with: a seagate USB drive, OS-X 1.5.8, VirtualBox 3.0.8.
References: http://www.virtualbox.org/ticket/4168#comment:9