Thank you @bmillham, I’ll give this a try.
Another brief power interruption happened again this weekend, no open files, and sure enough the drive ended up inaccessible, requiring the aforementioned fix again. I’ve now ordered a UPS (CyberPower ST625U) that should prevent the issue from happening again.
Strange thing is, I have two of the exact same external HDDs attached to a Z83 running a DietPi FTP/NFS server on the main floor upstairs, and these have never been affected by the same a power disruption. It’s a real headscratcher.