Git push error: "origin does not appear to be a git repository"

Here are the instructions from github:

touch README.md
git init
git add README.md
git commit -m "first commit"
git remote add origin https://github.com/tqisjim/google-oauth.git
git push -u origin master

Here's what actually worked:

touch README.md
git init
git add README.md
git commit -m "first commit"
git remote add origin https://github.com/tqisjim/google-oauth.git
git clone origin master

After cloning, then the push command succeeds by prompting for a username and password

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

Most likely the remote repository doesn't exist or you have added the wrong one.

You have to first remove the origin and re-add it:

git remote remove origin
git remote add origin https://github.com/username/repository