Gmail – save all the emails in one particular Gmail folder onto a disc/stick etc

Did you read the answers to your other question?

Per the link I gave in my answer to your other question:

If you absolutely need the old label structure, the only solution is to use an IMAP client to connect to both the old and new account and copy over folders/labels. But this solution can be tricky, because you can only copy over a certain (unspecified) amount of mail at a time. The non-Google program Gmail Backup might be able to assist with this process, but I can't vouch for its effectiveness.

Regarding contacts, from the same link:

Step 3: Export and import your contacts Login to the old account and follow the instructions on the Export Gmail contacts page. Remember the location of the saved file.

Login to the new account and follow the instructions on the Importing CSV files page using the file you just saved.

Just use labels and a filter.

1. At the top of your inbox click labels.
2. Click Manage Labels.
3. Click Create New Label.
4. Enter a name, let's say "Emails from my friend."
5. At the top of the screen to the right of the search click Create a filter.
6. In the from field enter your friend's email address, e.g. friend@example.com
7. Click Next Step.
8. Tick Apply the label: and choose "Emails from my friend" or whatever you named your label.
9. Tick Apply filter to x conversations below then click Create filter.

You will now have a label on the left hand side of your inbox. Clicking this will show all emails from your friend. It will also add the label to all the emails from your friend in whatever folder they are currently in (e.g. your inbox).

Have a look at changing the colour of the label to make it stand out. As an example this is what two of mine look like: