It was the boot drive, which housed all the sites.
Migration to a new drive is complete, so things are stable for now.
In the coming months, the plan is to migrate to a less monolithic approach, using Docker containers.
Due to an oversight, the web server was not configured in RAID1. And unfortunately, for technical reasons, neither is the replacement drive at the moment. But as we migrate web services to containers, we will certainly be using RAID1/5/6 going forward.