[Quoth]

USB Mass Storage was probably using an actual FAT32 partition, and a .prefix doesn't hide files in windows. Probably on Mac & certainly Linux you can "display hidden files" and "display backup files" the .files or .directories and backups end with ~ and are normally hidden, in the File Managers. "Emulating" is a strange choice of phrase by Amazon.

I'm not clear what your problem is?

With the Sage and Elipsa there are exported notebooks (can be any of *.txt, *.pdf, *.png, *.docx or *.html) most of these are "valid" files for Calibre but I'd not want to match them to the Library or import them to Calibre. I see these in Calibre if I pick "Device" instead of Library. I just ignore them. Likely png don't show?

If I've put ebooks or PDFs on via a different computer (Calibre or not) I might want to "match" to Library and "Add" if they aren't in it. That's rare.

Also other Kobos will have saved *.svg files from the sketch pad and unusually the Libra 2 seems to have a digitiser so the Kobo/MS/Ntrig Pen works on it. I've not investigated if these show up on Device View on Calibre.

Likely Kindle myclippings.txt shows?