Hi, MusicJohn!

Congratulations on your new keyboard. The PSR2000 has about the equivalent of one-half of a floppy disk available for "USER" memory. That 720K of space is shared by all the different files you have in the USER area. You may not have any other "songs" stored in USER memory, but you may have styles there, or voices, or multipads, or a different music finder database, or different voices. These all take up that space. This is all explained in some detail in the lessons on the PSR File system at http://psrtutorial.com . Here is a direct link to those lessons:
http://psrtutorial.com/Lessons/L_FileTypes/l_filetypes.html

Since you now have a PSR2000, you will be bookmarking the psrtutorial and going there frequently over the next year as you explore all the capabilities of your new keyboard.