As long as you have two copies of your data you should be fine with a USB flash drive and the data stored on your hard drive. But I would suggest spending another $15 to get an extra flash drive to have a third back up just in case.
I am also going to add one word of caution. Be very cautious when you back data up to not overwrite new data with older data. It is a very common easy mistake to make.
Edit: As long as you make back ups once every five to seven days you should be fine.