Running constantly

My drobo is still running after shutting down. The drives still rotating. The only way is to pull the power cable. Even when i put in the cable again, the drives directly start spinning. Is the 8D constantly running?
Is that normal?

My 5D3 started doing this recently. It seems to have started after I replaced a couple of 4 TB drives with 6TB drives (WD Red), that the Drobo insists are SSD drives.

Have you let it run and timed how long before it stops - or are we talking hours?
My 5D will run the disks after shutdown as it internally optimises data across the HDDs, something it needs to do more frequently if space is running low, but will cease after about 5-10 minutes by itself.