You can navigate into the folder structure where there will be an 'originals' folder or similar but I would just export them all to the new drive.
edit: the folder is called 'masters' now and of course these files won't contain any edits.
Is that using time machine or some other back up method ?
It's just where the files are stored on your computer, you can do with them what you want.
In a Finder window navigate to you r photos folder, right click on the Photos library and select 'show contents'. From there you can see how Photos stores your files and do with them what you like.
I'd still just export them to a backup drive though.