Does Github serves as an online external drive? How much available space storage if so?
This is not the intended purpose of github. It's intended for management/perusal of software source-code (and related assets).
Please also see this article: https://help.github.com/articles/what-is-my-disk-quota
For standard online storage, I would look at Dropbox, SkyDrive, Google Drive, Amazon S3, or an online backup solution.