If you noticed that your hard drives don’t spin down while PhotoStructure is running, you’re not alone.
PhotoStructure has several periodic health checks that run commands like df which also may spin up disks. These health checks run every minute or two, which prevents drives from spinning down.
The solution is to have sync and web determine when they are idle, and “hibernate” automatically.
Know that this is a known issue and that I hope to fix this soon.