cl_gui_frontend_services=>directory_create - Downloading to a USB Stick

Hi,

I'm using the Class CL_GUI_FRONTEND_SERVICES method DIRECTORY_CREATE to create a folder and then load some files into it. If I download the files on to my computer it works fine, but if I try to down load to a USB stick I get a DIRECTORY_ACCESS_DENIED error. Does anyone know why this happening. The requirement is to download to a USB stick.

Thanks,

Heidi