As it has already been mentioned in che’s answer about adding the remote part, which I believe you are still missing.
Regarding your edit for adding remote on your local USB drive. First of all you must have a ‘bare repository’ if you want your repository to be a shared repository i.e. to be able to push/pull/fetch/merge etc..
To create a bare/shared repository, go to your desired location. In your case:
$ cd /Volumes/500gb/
$ git init --bare myproject.git
See here for more info on creating bare repository
Once you have a bare repository set up in your desired location you can now add it to your working copy as a remote.
$ git remote add origin /Volumes/500gb/myproject.git
And now you can push your changes to your repository
$ git push origin master