With GH World Tour, a separate save file is created on the Wii's flash memory (300+ blocks per song). I forgot if songs are separate files or one big save for all songs. There is an option to move the songs to an SD (reg.) card. I don't remember if temporary WiiWare-style free space is needed on the main flash memory.
GH 5 added SDHC support, and requires all World Tour songs to be RE-DOWNLOADED if you were going to use GH5's interface. GH5 featured a brand new music store, whose songs are not compatible with the World Tour disc, despite both titles being released the same year.
Activision, get fukt.