cd /media/"External Hdd"
or (to display a text file with whitespaces in the terminal)
less /home/usr/"Word Space File.txt"
Or, if that doesn't work, a backspace might do the trick:
The double quote method works for me, both with directories and files: however, the lesson learned is that, if you intend to use the command line, try to avoid embedded spaces in Directory and file names. For instance, a better name for the drive in this case might be: External_Hdd (note the underscore!)