If I understand you well, you are trying to change remote directory (on FTP server) to another drive.
I'm afraid that neither FTP protocol nor FTP servers work with "drives".
There is a root directory defined on the FTP server and you can change your working directory into its subdirectories, but not to another drive.
I can see several ways to access directories on another drive, but all of them have to be cofigured at the server:
a) If you need to access files on the C: drive only, change your FTP root directory to some directory on the C: drive.
Setting of the root directory depends on your FTP server.
On most of FTP servers, you can define a specific root directory for each individual FTP user.
b) If you need to access files on both drives C: and D:, you can link desired directory from C: drive to the directory structure under your root FTP directory on the D: drive.
You can do it either by defining virtual directory on your FTP server (if the server supports it - most do) or by defining link on your filesystem.
For example, to link directory C:\data\public to D:\ftproot, you can use following MS Windows command on Windows Vista, Windows 7, or Windows Server 2008:
mklink /d D:\ftproot\public C:\data\public