I've also gone into my Kindle preferences and made sure the USB preferences say 'File Transfer'.
I've restarted my Kindle, Mac, and Calibre but it doesn't work. Meanwhile, my old MacBook Pro and Calibre still do recognize my Kindle. However, whenever I plug in my Kindle 7 (9th Gen), Calibre doesn't recognize it. I downloaded the latest Calibre and transferred all of my books onto it. I recently bought a brand new MacBook Pro (Big Sur OS).
