I offered to help a friend tag some of his (legitimately owned) MP3 files. It worked perfectly! Except, when I dropped it on his Zen m300, it all came up as “Unknown Artist”/”Unknown Album”. Rubbish! Oddly, some pre-existing files on the device were correctly recognised, and some were not, leaving me scratching my head.
After many (MANY) hours of tinkering, I went down to just two files. The working ones were ID3v2.3 tagged, whereas the non-working ones were ID3v2.4. I tried a few different pieces of software but in the end forced it to work with MP3tag (http://www.mp3tag.de/en/) – go tools, options, tags, mpeg and select ID3v2 ID3v2.3 ISO-8859-1 in the “Write” section of Tags>Mpeg.
Your mileage may vary but this fixed it for me!