It is a definite option.
I currently have two externals, I bought my first one because I only have a 80g internal drive, my second because i filled my 1st rofl..
Currently I use one for computer files, backups and music. The second I use for anime, manga and videos.
I occasionally reformat my computer....about every 6 months, and having all my stuff backed up to externals is very convenient.
This also applies if you get a virus, or your computer crashes.
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