...I want to see all the folders in which I may have photos - not just those under "My Pictures". How do I revert to this view? (I have 2 copies of Lightroom on 2 separate laptops. The one allows me to see all my folders, while the other does not.)
The folder panel only shows folders when you have imported images from those folders. (No import = No show)
It does not show other folders (in the way that a file browser does)
Click the small triangle icon beside a parent folder to open/close any sub-folders.
Click the small triangle icon at the end of a Drive header to open/close all folders.
This may be an answer to your question-
Right-click a topmost folder in the folder panel to Show a Parent folder. (Repeat until you see the Drive name location)
(Also Right-click a Parent folder to Hide that parent folder.)