You are very, very confused.
No, external storage does not mean redundant.
Drobo (and some RAID arrays) offer yes, an external storage solution, but that is not why they are unique. They offer realtime data protection in a RAID5 like array. Meaning it is protected in realtime against single drive failure. Meaning unlike a simple external drive, if one tanks, you lose nothing including operation of the device.
All the data on my Drobo is available and protected. I not only duplicate some of the data on my internal array for yes, double redundant protection, but I also store data only on the Drobo that is less critical but needs to be protected none the less.
External drives are for transport, backup, or limited capacity primary use. They can not offer the features in a Drobo or some RAID arrays.
If you don't know what you're talking about, ask questions, don't make incorrect statements.