I do not know of a hard disk for the G600. You may want to check Megafloppy from the italian company Liontracks; keep in mind, though that it will void your warranty.
As for lyrics - I could never display lyrics on my G1000 either, however I did not try very hard. You may want to check RAG homepage, http://www.daytonamusic.com/rag/
- they have a tips and tricks section.