Your first time using draw.io with Dropbox
When you select Dropbox as your storage mechanism in draw.io the next splash screen will be
Click the Authorize button to allow draw.io to access a specific folder in your Dropbox, usually Apps/drawio. If you want draw.io to forget your Google Drive authorization later on, when logged in your name appears in the top right pull down
Selecting "sign out" will de-authorize your current login to Dropbox (it won't disconnect the application from your Dropbox permanently).
The first time you authorize Dropbox you will see a dialog like
When you click allow draw.io will have write and read permissions to only that folder. This means that all your files not related to your diagrams are completely secure from draw.io.
Linking draw.io to Dropbox means that draw.io can read basic information about you, including your email. However, none of this information resides anywhere except your browser and Dropbox's server, it is never passed up to draw.io servers.
Once you are authorized you'll see the new/open dialog. See the Basic Usage section for a description of how to use this.
If you decide to switch default storage mechanism at any later point, navigate to the application web page and you'll see this dialog. Click on "Not using Dropbox" and your storage selection will be forgotten and the initial storage splash displayed. If you want to change Dropbox user logout using the pull down at the top right.
The orange draw.io logo in the top left will take you to www.dropbox.com. Clicking the title of the diagram allows you to rename the Dropbox file.
Dropbox files are autosaved (you can switch this off under File -> Document properties). If you have a native client with notifications of file changes enabled, this can be annoying. In this case switch off the autosave and perform manual saves when required. Dropbox don't, at the time of writing, provide a mechanism to avoid the notification from triggering during autosave.
New, save, save as and make a copy are self explanatory. Open invokes the Dropbox file picker, which has seperate permissions from the draw.io application. You can load anything into draw.io, from any folder, but be careful doing this since you still can only save to the permitted folder. Thus, if you open a file in another folder and perform File -> Save then the file will be saved in the Apps/drawio folder instead, leaving the original unchanged.