Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your holder made a claim about where it lives, and the claim turned out to be false. Let's look at it. When you write which ext4 and a path, you are telling rush: my data rests on an ext4 disk, at this place. Before it writes anything, rush asks the kernel what is truly mounted at that path, picking the deepest mount that contains it. If the kernel answers tmpfs, the memory-backed temporary disk, then your belief and reality disagree. Maybe you are saving something important that you think is on a permanent disk, and it would vanish at the next reboot. That is exactly the kind of lie rush refuses to carry. It stops by name, and the second slot lays out the whole story: the path, what it really rests on, and what you claimed. The fix has two options. Either move the path to a place that really is on the filesystem you named, or change the which word to match reality, if reality is what you actually want. Either way, you now know where your bytes go before they go there.
Text from the Rush engine's error catalogue.