If it's just extra storage space you need, get a USB external drive. They are only slightly more expensive than internal drives, and can be stored separately from the pc when not in use. Any backups made to it would still be secure if the pc was affected by lightning strike, power surge etc.
If on the other hand you need more room for programme files etc, fit an internal one. It's so easy even I managed it. Most HDD manufacturers have software and tutorials which allow you to clone the old drive onto the new one so you end up with just one, much bigger HDD. Alternatively you could simply fit the new disk as a slave and keep the original as it is.
I agree with keef66 on both points. Fitting a bigger primary HDD (and poss faster) would be a great idea and then you could still fit your original as slave (you know it's compatible) for backup purposes (shall we sing Blondie's Acronis together?)
On the external front, the Lacie unit I have is fantastic for holding all my data, and I can take it anywhere and plug it into other machines (at work for eg). It is however rather louder in its whirring than my silent laptop is. It sits under my desk but I can still hear it. Not a terrible thing but noticeable and I switch it off when watching dvds.
Just some extra points to consider.
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