Send your thank-you letter to Apple.
First of all, when you’re ripping albums you can go direct to mp3. Importing Settings shows up in iTunes when it lists the tracks of an album–down on the lower right in my Windows version, but Apple likes to move things around. That way you can rip directly to mp3 instead of converting. Use Custom and 320kbps for the mp3 settings–lower bitrates were for the days of more limited storage.
Now about your itty-bitty files…
The Unknown folder you are seeing is not really a folder. There is no folder called Unknown on the Sansa. Unknown means unknown artist because the Sansa makes Artist, Album, etc. listings from information gathered from ID3 tags, electronic labels in the files.
Now, those files that start with underscore (actually period underscore) aren’t really mp3 files and don’t have tags–so they pop up as Unknown. If you really like to go by actual folders, you can scroll down to the end of Music and see Folders–those are actual folders, not tags.
But what are those Unknown thingies?
Well, when a Mac rips a CD, it creates the music files (for example 01-First Song.mp3) but also makes a Finder file that the Mac uses to locate things, and the Finder file for that song is ._01-First Song.mp3. (starts with period underscore). Then, because the Mac calls the finder files .mp3, the Sansa thinks they are music files. So cute.
You can just get rid of all of the Finder files from the Sansa, which doesn’t need them. If you’re on Apple your Sansa is probably connecting in MSC mode and you should make that permanent–Settings/System Settings/USB Mode/MSC. (MTP is to connect to Windows Media Player, Auto switches between them and makes trouble).
Now look into your music folder on the unit or card and do a search for files with ._ in them. Maybe you have a setting somewhere in the MAC to show hidden files or something–try Help. The Finder files are often in folders called MACOSX within the album folders, so you can also search the Sansa for MACOSX. All the Finder files are 0kb, so you can also sort by size. When you find them, you can happily delete all of them from the Sansa. DS_Store files can also be deleted.
If your Mac just refuses to show the Finder files-I’m not sure, I’m in PC world–then make sure USB Mode is MSC (as above), find a pal with a Windows computer, connect and search and delete them from the Sansa.
Mac people have written here about a program called Hidden Cleaner that does this automatically–you might try it.