External HD has disappeared from Finder
You have only removed the visibility on your desktop.


Go to Finder (The smiling blue man in your Dock in the lower part of your screen)
Then press Command, or go to Finder (menubar) and choose 'Preferences'
Now choose 'General'
Tick the checkbox that says "External Drives"


You can do the same for the Finder Sidebar


Within the Finder Preferences choose 'Sidebar'
Tick the checkboxes in the Devises section of the devices you want to see


Somewhere within the evolution of Finder they added this option, I am almost certain is wasn't added yet in OS 6 ;) With this feature they created a cleaner desktop and some confusion, I must admit. 

Another easy way to check which drive is mounted is to open "Disk Utility" located in Applications/Utilities/

Good luck with your drive.