How can I disable Finder notifications?
From what it looks like there isn't a decent way to do this without unloading the entire Notification Center and any other notifications alongside with it.
However, as @Tetsujin noted and as many other people would also concur, for the sake of your drive and how macOS uses it I'd personally recommend just ejecting it for the sake of your data and not to risk nuking that drive even if the chances are small; annoying but necessary with how the file system deals with write caching.