On Research Drive the user has a personal account. Collaborations do not play a role, here. Nevertheless, sharing data is one of the key-features of Research Drive. If you like, you can label Research Drive as the "scientist's Google Drive or Dropbox".
External storage is persistent storage that a user can create for a particular Collaboration. The procedure for creating an external storage volume is very similar to creating a workspace. External storage is fast enough to be used for computing tasks that require a lot of recurrent disk access.
The term Datasets in SURF Research Cloud designates any implementable way to attach a specific set of data to a workspace. Just like components, datasets are Ansible scripts that realize a way for the given workspace to access a given set of data. This might be a simple copy process, mounting remote storage or other means. Through the concept of Collaborations, there is fine-grained control on the access to a dataset.