Yes, you can. I have successfully replaced the original hard drive with a 2TB hard drive I purchased. Just make sure you buy the correct type of hard drive. I recommend using the article Kenneth has linked above in the comment section: http://www.ign.com/wikis/playstation-4/H...
I backed up my saved data on the PS network instead of using an external hard drive and that worked great.
Enjoy the extra storage!
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