What is the difference between HFS+ and EFI System Partition (ESP)?
The ESP is normally used for the boot loader on ‘normal’ EFI systems, however on a Mac the ESP is not—it is usually empty, but has been used as a staging area for firmware updates.  A Mac will initially look in the ESP for anything to boot from, but if it is empty or non-existant, it will look for the root file system to boot from.