When you write a new hub, there appears on the top bar (black) 3 buttons after the HubPages logo: (1) Preview, (2) Save Unpublished, and (3) Publish.
If you want to save a draft of your article, simply click #2 ==> Save Unpublished. However, I'm not sure if HubPages save our articles automatically. It might therefore be necessary to regularly save unpublished your article to make sure you don't lose any text when you get disconnected.
Although it is best to write an article in our PCs first. But I do write them directly here in HubPages.
Wish you the best.