Unfortunately, I wasn't even able to capture the USB drive this time using the same method I have always used to capture a USB device in VirtualBox.
Now I'll see if I can work out how to get the screenshots into this post.
1. The flash drive is mounted in Linux.
2. What happens when I try to capture the flash drive in VirtualBox.
3. What happens when I hover over the "USB device" icon in VirtualBox.
I opened another VirtualBox client to make sure that it wasn't just this client that is playing up. The same thing happened in the Android 4.3 client as well.