Dear Aunt TUAW,
I am writing you because I think you may want to publish this on your column, as a tip to your trillions of readers.
On Leopard, Apple included Notes within Mail, an app I use exclusively. I wrote many notes on Mail, including to-do reminders and background notes.
Enter Mountain Lion. Mail upgraded and took a dump on my notes. *pffft* They were gone, kaput, with no way to access them.
Fortunately, I discovered that their folder was not erased. Navigate to ~/Library/Mail/V2/Mailboxes and locate Notes.mbox folder inside.
Open this, and all its subfolders, and look for numerical files with the extension emlx. These are your notes. When you double-click, Mail opens them, allowing you to copy the notes to the new Notes.app.
I hope my experience will help others! Please spread the word.
Your loving nephew,
Auntie hopes your how-to offers a handy solution for her other nieces and nephews. Be aware that the notes may be buried under subfolders in the Notes.mbox folder. Of course, if your notes were syncing to iCloud then they'd have been backed up and safe, which is a pleasant thought.
In principle, your notes should migrate over when you upgrade from Lion to Mountain Lion, but this is helpful if for some reason they don't. You may also be interested in checking the three-pane view in Notes (click the three-pane control at the bottom, to the right of the plus button) as that will display your different Notes accounts on the left hand side.