Computer/Hardware: What's the difference between a dvd writer and a portable external hard drive?

a DVD writer

- stores data on a DVD disc

- the disc can then be transported from one DVD player to another

a portable external hard drive

- allows data to be stored on the hard drive

- the hard drive can then be transported from one computer system to another

- DVD writer is much slower and storing and retrieving data

- DVD, if data is stored in DVD audio or DVD video format, can be played on any DVD player (even one not connected to any computer system)

- an external hard DISK drive needs power cord (and a socket at each location it is used)...but a DVD disc does not

- - - but note that memory drives, such as a "thumb drive", only need an operational USB port

A big one which should be obvious. 

Are you being serious?

A DVD Writer allows you to 'burn' music/movies/programs, etc. to optical media, whereas a portable external hard drive allows you to 'copy' music/movies/programs to it, and then connect that portable external hard drive to other computers/TVs, etc.