If you want to know the free space in C drive, just enter the following code:
RFs iFs;
iFs.Connect();
TInt err = iFs.Volume( volinfo, EDriveC );
TInt64 space = volinfo.iFree;
iFs.Close();
"iFree" is the amount of free space on the disk in bytes.
You know how to get the D drive's free space :P
Remember, Volume() will return KErrNotReady if the drive contains no media.