Is the light solid red or blinking red.
Solid Red would indicate the Drobo is full ( I will come back to this)
Blinking Red means the drive is bad
Coming back -
Keep in mind the Drobo uses the largest size drive for redundancy. If you have dual disk redundancy enabled then the two largest size drives are used for redundancy. This knowledge base article will help to explain:
What is the capacity lower than expected?
Something else to keep in mind; The largest size drive we have tested in the Drobo S is 4TB, the Drobo S also has maximum usable space of 16TB. Once this limit is exceeded a second volume will be created.
Since we no longer manufacture this drive, you may want to look at some of our newer products. Last, ensure you are on latest firmware for Drobo s. https://www.drobo.com/drobo-downloads-2/
Maximum Volume and Drive Size for Drobo
Hope this helps.