Thanks banjobama.
I'm not sure if it's from the SD or internal memory to be honest. I think it may be from the SD card. It's just a folder on my computer desktop called "PRS600 backup". It might even be a combination of both. So am I safe to delete everything else except the media folder? I don't want to keep settings and things like that, as long as my books and notes remain, then I'm happy.