Apple - How to enable the sidebar in finder?
For me, the solution is that I need first (by Apple mis-logic) to hit:
"View / Show Toolbar" (⌥⌘T);
…then "View / Show Sidebar" (⌥⌘S) becomes ungreyed.
Carefully move the pointer to the left side of the finder window border till the cursor changes to arrow pointing to the right.
↦
Hold and Drag to the right.
If that did not work you need to restore the finders preferences.
In terminal run
rm ~/Library/Preferences/com.apple.sidebarlists.plist
and
rm ~/Library/Preferences/com.apple.finder.plist
Restart.