Sorry, on demand is not a good way to state this, it’s just how my weird mind thinks of things. By “on demand”, I mean, like you are actively using it to store something or view something. If you’re not intentionally doing something with it, the drive should be completely idle. That’s more of a target than a requirement, though. It’s a way to keep storage drives tidy and not littered with temporary cache files, or databases used to store runtime state by various services. It’s just a strategy I like to take, to keep bulk storage separated from the applications and services that use it.

Even if a usb drive is intended to be permanently attached, it should still be treated as a temporary component. The reason is so that if something happens and the drive is disconnected, it limits the disruption to the system. You lose your media and documents until it’s reattached, of course, but the computer keeps chugging along happily.

If you use it for writing log files, then its loss can disrupt those services (and also prevent the problem from being reported). Also it’ll be constantly making noise, which can be annoying.

That’s my reasoning, anyway, you might prefer it done differently.