Check the card with H2testW. You may have gotten a counterfeit card, a smaller card gimmicked to look like 32GB to Windows.
If H2testw likes it, then Format in the unit. (Under Settings/System Settings) and try to reload. That will erase all your added content–but it’s not working anyway, right?
Here’s an explanation of allocation units, but Sansas generally format to FAT32 so you can ignore it anyway.
Otherwise the only reason the unit shows Unknown is that it doesn’t see or can’t read what’s in the tag fields–Album, Artist, etc. Have you looked at the tags in mp3tag? ID3v2.1 won’t be read–and I know you said they are ID3v2.3, but doublecheck.
Under Tools/Options/Tags/Mpeg you can set mptag’s Write default to ID3v2.3 ISO-8859-1, which the Sansa likes. ISO-8859-1 is Windows text encoding. Highlight an album’s worth and Save them, and transfer them over, and see if the unit can read them.