I am running macOS Sierra but I have /nix on an external SSD which is formatted as Case-sensitive Journaled HFS+.

According to @shlevy, Nix assumes that Darwin implies a case-insensitive file system. The Darwin hacks were causing problems for me when running nix-build in a Haskell project due to a conflict between a package named Compactable and another named compactable. Adding use-case-hack = false to my nix.conf fixed the issue.

Rather than make assumptions about the file system, Nix should determine whether the file system is case-sensitive.