this looks bad - have any drive SATA-errors too? From plain data given, i would "sdf" change first, then "sdc", then "sdb" ... and may be use anonther vendor/type of harddrives ... btw. which drives do you use actual?
Thanks for the guidance. Can't check for SATA errors right now (none that I'm aware of), as the server is off at home (figured it would be best to switch it off until I could start drive replacements). They're all Seagate Barracuda 3TB drives. I've already replaced one of the others previously, and I have a new spare ready and waiting. I was wondering whether I'd be better off switching to a different model, for the next swaps. I'd read that Barracuda weren't intended/good for always on servers, and had seen Seagate IronWolf drives recommended. Any thoughts on that?