External Hard Drive - Essentially a large USB Flash Drive?

[mthwkrft]

Before purchasing an external hard drive I was wondering if it was basically a large Flash drive or if like my computer's internal Hard drive, if it has to have an OS installed, if I reformat my internal hard drive must I reformat the external as well?

 

[Our Mutual Friend]

pretty much. its like the flash drive is a fir cracker and the external hard drive is like a nuclear bomb