H&G Sync is the game's launcher/updater. When you enter the game's lobby it will start and then check if you have the latest files. If it discovers that a new map has been added or updated it will download the necessary files automatically.
Pr. default H&G Sync stores the game's files in the temp-folder. Unless you have changed it yourself in your windows settings this folder is on your C-Drive.
If you want to move the files to another drive select "File - Change Install Directory" - H&G Sync will then move the already downloaded files to this new folder and in the future download files there. When you have moved the files it is a good idea to do a 'verify files' to check that everything went according to plan.
If you run into problems with H&G Sync please let us know by opening a support ticket at support.heroesandgenerals.com.