Looking at the docs http://doc.qt.nokia.com/qtmobility-1.2/qsystemstorageinfo.html you can see:

availableDiskSpace( const QString & volumeDrive )
getStorageState( const QString & driveVolume )
totalDiskSpace( const QString & volumeDrive )
typeForDrive( const QString & driveVolume )

Notice how it alternates between "driveVolume" and "volumeDrive".

Unless there is an actual difference between these two, you should pick one and use it consistantly.