a.rb

/0/ruby/mail:bf4/correct_deps $ appraisal rake
WARN: Unresolved specs during Gem::Specification.reset:
      rake (>= 0)
      thor (>= 0.14.0)
WARN: Clearing out unresolved specs.
Please report a bug if this causes problems.
>> BUNDLE_GEMFILE=/Volumes/Shared/0/ruby/mail/gemfiles/mime_types_1.16.gemfile bundle exec rake
Could not find gem 'ruby-debug (>= 0) ruby' in the gems available on this machine.
Run `bundle install` to install missing gems.
/0/ruby/mail:bf4/correct_deps $ bundle
Resolving dependencies...
Using rake (10.1.0)
Using bundler (1.5.0)
Using thor (0.19.1)
Using appraisal (1.0.0)
Using diff-lcs (1.1.3)
Using json (1.8.0)
Using mime-types (2.3)
Using mail (2.6.0.edge) from source at .
Using rdoc (4.0.1)
Using rspec-core (2.12.2)
Using rspec-expectations (2.12.1)
Using rspec-mocks (2.12.2)
Using rspec (2.12.0)
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
/0/ruby/mail:bf4/correct_deps $ appraisal rake
WARN: Unresolved specs during Gem::Specification.reset:
      rake (>= 0)
      thor (>= 0.14.0)
WARN: Clearing out unresolved specs.
Please report a bug if this causes problems.
>> BUNDLE_GEMFILE=/Volumes/Shared/0/ruby/mail/gemfiles/mime_types_1.16.gemfile bundle exec rake
Could not find gem 'ruby-debug (>= 0) ruby' in the gems available on this machine.
Run `bundle install` to install missing gems.

Did appraisal generate any lockfiles in the gemfiles dir? If not, try running just appraisal first.. it should bundle all those gems. If that works, then run appraisal rake. I considered adding that to the instructions but wasn't sure if it was just me as it wasn't in the appraisal docs.