Your HGST Hitachi is CMR, so no problem there,
For upgrading, I was referring to larger HDDs in the future as your data increases over time. Hopefully there are a few more years for your 5C ahead!
To clean your Drobo, fully shut down and disconnect the power then mark your HDDs in order before you extract them. A small domestic vacuum cleaner and soft moist cloth wipe down should be sufficient to clean the unit. Pay attention to the front cover - this you can clean more regularly as it is the inlet for air drawn into the Drobo carcass by the fan, When clean, reinsert your HDDs in the same order, reconnect and restart.
OK!! I’m all set. The new HDD is in, the OS is 12.3.1, and the external drive has the Drobo backed up. I didn’t have a problem with the system preferences, although I didn’t see 2 files specific to the Drobo Dashboard in Full Disk Access (just one, I think, which was already checked).
Thanks so, so much for all your help!!
Now I can print some pictures before I put the thing away while we are gone.