After some time, I am starting to run out of disk space in my development machine (only 128GB). For this reason, I have decided to move local cache stores (.npm, .m2, .ivy2, etc.) to an external drive.
I switched recently to Nix for Haskell development after experiencing the well known "cabal hell". I haven't found a proper way to change the Nix store location, though.
Is it possible?
Yes, but you won't be able to use the binary packages (everything will be compiled from scratch). To bootstrap, see https://nixos.org/wiki/How_to_install_nix_in_home_%28on_another_distribution%29
A better idea might be to make /nix be a symlink to a directory on your external drive. Have you considered this? As @jarandaf mentions below, /nix cannot be a symlink.