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An external drive is exactly like your internal hard drive. It has spinning disks and read/write heads. There's no need to reformat an external drive when you format the internal drive.

There are hard drives that are just like flash drives now. They are called SSD (Solid State Drives). They are expensive. You could use one as an external drive if you wanted, but the best use of them is for a primary internal drive because of their speed.

No, they are not the same. An external hard drive is exactly the same thing as whats on the inside of your computer, just packaged different'y so it can sit outside the tower (or laptop). But, you will not need to reformat the external if you reformat the internal. And no, it doesn't need an OS on it. The OS is installed on your boot drive, also known as C:\ drive. You COULD put an OS on the external if you wanted, like Linux. But you don't have too. Also, I wouldn't recommend moving the external hard drive around too much. Some are kinda tempermental.

You can think of the external hard drive like a usb flash driver where you can store whatever you want and it is independent of your internal hard drive, although physically, it functions like an internal hard drive.

So to answer your questions, no it doesn't have to have an os on it, and you don't have to reformat the external when you reformat the internal.

external hard drives are more dependable for storing data compared to flash drives. Flash drives fail more easier than hard drives and should not be used to store valuable data such as pictures, documents etc...

Flash drives are best for transferring data and not storing them.

External hard drives do not need an operating system for storing data.

No you don't need to reformat the external hard drive when reformating the internal drive .They are independent to each other .

If you install a program on the external hard drive remember that the registry files are in the internal hard drive for that program. If you reformat the internal hard drive you will loose all the registry files for the programs you installed in the external drive . Your programs will not work until you re-install the programs so the registry files will be replaced.

No it is not like a flash drive. It is simply an additional hard drive. If you format your internal you do not have to format the external. You do not have the OS to be loaded on the external.

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