After i dragged the file from the main computer HDD into the portable HDD the computer said copying and the the file on the main computer seemed to remain the same it was still there in its original form and another copy of the file was on the portable HDD
Hmm... that's strange. Usually when you right click your mouse and hit "copy" on the original file and then go to your USB HDD and right click and hit "paste" it makes a copy of the file there and the original remains in it's original folder. But when you drag you move the original file to the location where you dragged it to.
actually i don't think that's true. all other relevant stock settings at default in windows, whenever you drag-click-drop a file from your c: to external hdd or flash drive (or vice versa) it always copies it over. this happens when a file is transferred from one drive to the other, but not when a file is moved around within its own drive partition.