Formatting will not erase the bootloader-modified firmware from the Rockbox installation. You have to manually re-install a new copy of the Sandisk firmware. You can find and download a copy in the Sansa Firmware Thread.
One other point when using a Mac . . . I believe you need to empty your “Trash” (on your Mac) while the player is still connected to fully erase them from the player. I don’t use a Mac so don’t quote me 100%, but that’s more or less what I have read.
