I realised shortly after posting this that I'd gotten the disk mentioned in the error confused with the name of my cache drive. I thought I'd deleted this post (Moderation Actions > delete), but I guess it didn't work.
Looks like Unassigned Devices thought that my external drive was plugged in when it wasn't (and that it needed a TRIM), so I rebooted my server and manually ran a TRIM and it worked without error.
Sorry for wasting your time.
Mods: feel free to delete this topic.
jademonkee's post in TRIM no longer working on Samsung 870 EVO was marked as the answer