Resolved USB Memory Stick vs USB Flash Ext HDD

I am going to buy a USB Flash Ext Hard Disk 300Gb for data storage. I have some confusion in my mind which need to be cleared.

1) What is the difference between USB Memory Stick and USB Flash Ext HDD?
2) Usaully we need to format USB stick after some days/months due to virus atttacks. So same could we do with Ext HDD or it is safe?

Any other instruction/information for Ext HDD will be highly appriciated.

 

I have a couple of Thumb Drive or Memory sticks what ever people want to call them and they are 4 years old or so and use them all the time. Never had to format one. I usually just delete what I don't need (if I don't have enough room) and add to it what I need. I guess if you thought that you had a virus, you could scan it.