Everything shoud be visible to Nickel unless you have a seriously twisty setup (i.e., a base library *inside* a hidden folder), but that would imply knowing what you were doing (which usually involved *not* letting Nickel see stuff), so, you wouldn't be here asking this question

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So, the most obvious answer is simply: Nickel doesn't rescan for new content outside of in-nickel USBMS sessions. But,
NickelMenu allows you to do that on-demand instead, so, do that

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