When I try to access an external USB flash drive with my running Ubuntu in Virtualbox, by going to the Devices -> USB section in the Virtualbox menu, it complains that the device is busy with a previous request (gives me a UUID number and some more details). Not sure if this is because it's automatically mounted by Wwindows and I need to do something with it before being able to access it with
This error indicates that the drive can not be unmounted from the host, most likely because it is accessed from an application there, or any file operation to the flash drive is pending.
Since your host OS is Windows we can not really tell what might be the cause in your case, and how to resolve it (virus scanner?).
Workaround: add a filter in the "Ports" tab of the virtual machine settings, then unplug the disk, start the virtual machine, and plug it in again once the VM has booted.
I just had the same problem. When I unplugged the device from the (blue) USB 3 port and plugged it into the (black) USB 2 port, it worked. Seems like VirtualBox doesn't like USB 3...