This has been driving me crazy for quite a while and I haven't been able to find the option to fix it.
If I currently have a location selected in the Folder Tree that's on a local drive, the section "This PC" is visible.
As soon as a I pick a favorite that's on a network drive, "This PC" disappears off the Folder Tree and I'm not longer able to jump to a drive unless I pick another favorite that's on a local drive.
You've probably set Preferences / Folder Tree / Contents / Start Folder Tree at... to Current Drive.
Change it back to Desktop.
Having the tree locked can also complicate things. If it's not locked on purpose, unlock it by clicking the lock icon next to the tree's close button, so it is open. (If you're using a layout, re-save the layout to save that change. If using the default lister, it updates automatically when a window is closed, by default, but if that has been turned off, use Settings > Set as Default Lister to update it.)
Thank you. That was driving me crazy. 
