With disk it means your Hard Disk Drive (also refered to as HDD).
For me it is not comprehendable why you should not be able to download the offline installer onto your HDD.........because that's exactly what you should do. It is always better to install any program using the offline installers rather than the online installation process.....
Is there any error message appearing when you try to do so?? If so, could you tell us what it says ??
