DROBO FS does not support drives from other Drobo products unless you want it to reformat the drives and clear all your data. I believe the manual shows a procedure to reset the Drobo FS and wipe all the data from the drives.
Thanks for the suggestion, resetting (pressing the reset button on the back) worked!!! I had “assumed” that when I plugged the drives in from the Drobo S, it would just reformat.