Whether you call it the 3-2-1 Rule or the Rule of Three, when it comes to good storage of your data you need to plan for a basic common scenario for data loss: the destruction of two storage instances in the same physical location and the loss of any given storage instance because of hard drive or cloud storage failure.
To avoid this, you need to have your data in two different physical locations and on two different media. This is accomplished by having your data saved in three places: on two hard drives and a cloud, or on a hard drive and two cloud instances.
But remember that a good storage environment needs updating. You data is probably constantly being changed and versioned. So a good storage setup also includes robust provisions for tracking versioning and storing older instances of files. Consider setting up an automated backup schedule, a common feature on many external hard drives, and look for cloud services such as NYU Box that track file changes. Most importantly, work out a file version naming system and document the contents of data files as you go.
For more information about file storage options and practices at NYU, consult the Research Data Management library guide.