Mac storage basics · External storage and adding space
How do I see how much space is left on my external drive?
Short answer: Select the drive in Finder and press Command-I, or open it with Finder's status bar shown. Trashed files keep using its space until you empty the Trash with it connected.
Select the drive in Finder's sidebar and press Command-I to see its capacity, used and available space, or turn on View → Show Status Bar and open the drive. Disk Utility shows the same when you select it.
Remember that files you trash from the drive keep using its space until you empty the Trash with the drive connected.