It seems the TM drive was formatted Case Sensitive!? And this drive is Case Insensitive!?
So with case Sensitive we could have 2 different folders like...
But with Case Insensitive they both appear as the same name & only one can exist.
I'm not sure the fix for yjis, but it seems the current one would have to be renamed or something. And no idea how iPhoto would react to that, possibly no problem if not running while you do this.