[Old Pedant]

Since this is for windows, there is a much easier way if you are willing to use MSIE only:

function findDriveThatIsPlaying( )
{
    var drives = "BDEFGHIJKLMNOPQRSTUVWXYZ";
    var fso = new ActiveXObject("Scripting.FileSystemObject");
    for ( var d = 0; d < drives.length; ++d )
    {
        var driveletter = drives[d];
        if ( fso.FileExists( driveletter + ":\\drive.js" ) )
        {
            document.cookie="yourdrive="+driveletter+"; path=/";
            return driveletter;
        }
    }
    // if you get here, no "drive.js" found on any drive
    // what do you want to do???
    return null;
}

I think that *HAS* to be *MUCH* faster than trying to get JavaScript to load a file and run it.