Is this the same sort of activity you were seeing @Moan before you disabled SMART ?
Sorry, I don't remember what it exactly shown, but "Loaded: loaded (/lib/systemd/system/smartd.service; enabled" was displayed as well.
When I stopped this service it shows disabled. Now disks never wake up when not needed.
I hope it solved your problem.
By the way, I store Plex database on the main SD card, not the external disk. If you have a different configuration it can cause those wake ups.