How do I download my data automatically to external harddrive programatically.

Do you mean that you have 750GB of data on Dropbox that you want to copy to an external harddrive, or 750GB of data on an external harddrive that you want to upload to Dropbox?

In either case, if you want to do this programmatically, you can use the HTTP API to accomplish both. Take a look at the links below, which will direct you to the relevant API documentation. They also include example curl commands.

/download: (https://www.dropbox.com/developers/documentation/http/documentation#files-download) 

/upload: (https://www.dropbox.com/developers/documentation/http/documentation#files-upload)

Note that if you have the Dropbox client installed on your computer, you can also do this manually via your computer's file system UI.