To display Disk Space, Photo Count, or Status (on/offline) in the header for any drive, Control -click (Mac) | Right -click (Win) on the volume header in the Folder panel and select the desired option.
Lightroom also provides a hint as to the available disk space on a drive by displaying a color-coded Disk icon in the Folder panel’s header. Green means that you have a significant amount of free space, yellow hints that the space is becoming limited and red warns that the drive almost full. When the header of a drive is gray, the drive is off-line.
In addition, clicking the + (plus) icon on the Folder panel header provides three choice for displaying the Root folder’s path (the Root folder refers to the top most parent folder(s) in the panel). Choose from Folder Name Only, Path from Volume, or both Folder and Path.