Windows Virtual Machine in VirtualBox on Mac OS X does not recognize any USB devices (i.e. flash drives etc.) that are listed under “Devices” and they are greyed out and unable to mount (refer to picture below).
This can be easily resolved by just un-mounting the drive from Mac OS X to allow Windows to have access to the device and then simply going to “Devices” in VirtualBox and clicking on the device you need to mount. Step-by-step illustrated instructions are below.
Step 1 : After inserting USB device (flash drive in this case), it is greyed out and not accessible to mount in VirtualBox.
Step 2 : Next to your device click on the eject icon in Mac OS X Finder.
Step 3 : Now you can return to your Windows VirtualBox and click on “Devices” on the menu bar and go to USB Devices and your device should be highlighted black now, click on the device and it will now be available to you in Windows.