On my early 2009 Macbook (white, not pro) I've been running Mavericks and Leopard 10.5 (for PowerPC apps). I decided to finally upgrade to El Capitan. On Mavericks I had no password. Now that El Capitan is installed, It goes to the log in screen and tells me to enter a password. I don't have one. I put in nothing, says its wrong. I put in my itunes password, says its wrong. Booted into the recovery partition, it freezes. I boot into Leopard, it no longer boots fully after upgrading the other partition to 10.11. I have no idea what to do --edit-- Leopard 10.5.8 decided to boot up after three more tries. Running disk permissions / fixing. Going to create a 10.11 USB with disk utility. Hopefully you can still change a password through that. If not, I'm just screwed I guess. Thanks Apple. Note to people reading this, Don't upgrade to El Capitan without a user password. Better yet, don't upgrade past 10.9 at all. Hell, stay on Snow Leopard.