Drobo not recognizing new drive

one of the indicators turned to yellow which meant that the storage space is running low. i have 5x 1TB drives. i bought a new 3TB WD red and installed it. after it finished the bay below turned yellow.

when i opened the drobo dashboard it showed the 3TB drive but didn’t increase the storage space.

i have a drobo S

could you please help me with this?

This should help explain:

Capacity Calculator at: http://www.drobo.com/resources/drobolator.php

Thanks a lot. I didn’t realize that I have to buy at least two drives. Thanks again