External diff seems broken in sourcetree 2.7.6-177 (Mac): when I try to external-diff nothing seems to happen, and I have had the same issue with 2.7.5.
If - on the other hand - I replace 2.7.6-177 with 2.7.3-169 then it works; to me this indicates it is something with the 2.7.5 and 2.7.6 releases rather than my configuration.
To replicate, simply modify a version-controlled file, and open its repo in SourceTree: SourceTree will show the modification in its diff-pane, and yet r-clicking the file and selecting 'External Diff' does nothing; i.e. no error-message is shown, nor does the external-diff tool get launched/opened.
SourceTree pref -> Diff: External-Diff/Merge setup as follows:
Visual Diff Tool: Custom
Diff Command: /usr/local/bin/bcomp
Diff Command Arguments: $LOCAL $REMOTE
Merge Tool: custom
Merge Command: /usr/local/bin/bcomp
Merge Command Arguments: $LOCAL $REMOTE $BASE $MERGED