I've just had some problems with a disk that was formatted with HFS+ case sensitive while every else by Apple was non case sensitive.
After a long time moving things away from my biggest disk, partitioning and formatting non-case-sensitive, I've found out that Time Machine does format "case sensitive"
Why Apple wants case-sensitive only for Time Machine??? What's the technical advantage? I've just invested a long time fixing sync problems with my disks because of different case sensitivity problems