how do I tell it to start in the D drive?
Close eM Client and move the existing database to the new location. Your database is the complete C:\Users\username\AppData\Roaming\eM Client folder. You may have to enable hidden items in Windows Explorer to navigate to that folder.
Once the database has been moved, open eM Client again and cancel the setup prompts. Go to Menu > Tools > Settings > General > Storage, and select the new location.
Restart eM Client.
If the D drive is a slower RPM mechanical disk, as usually found in laptops or budget PCs, you will notice a significant performance drop when using eM Client.