Oracle Virtual box (v 4.3.4 r 91027) installed on a Windows 7 x64 throws a Failed to attach USB device STMicroelectronics STM32 STLink to the virtual machine .. .. is busy with previous request..
As I spent somt time googling the answer, here is what helped me out: https://forums.virtualbox.org/viewtopic.php?f=6&t=59303
Basically you have to add your device and a device filter.
(This steps I get from here: superuser.com/questions/306172/how-can-i-make-my-virtualbox-vista-see-my-usb-device-ipad)
Here the video that illustrates step by step, how I fix this problem: USB device is busy with a previous request
Credit to: youtube.com/user/tutorialnacional