The fact that it won't open usually means an application is using it. The way to
make it accessible is to shut any applications that may have accessed it. Check
your system tray and if needed to System Monitor instead of re-booting.

In VBox you can set it up so that is uses Passthrough in the settings, making it
accessible in both, but not at the same time. You can change whether host or VM
has the drive while the VM is running by clicking on the status bar in the lower
right of the Vbox appliance window, assuming that you are not in Seamless mode.
If this is the case, you will have to go into window mode. Or you can do this in
the menu under Devices.