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I believe this is completely subjective, and really just depends on your usage. For me, 128GB is plenty. I don't store media on the internal disk, no need to. Just the OS and apps, and so I have over 50GBs free and I've had my rMPB for a year. I have a 128GB SD card permanently installed and thats what I store media on when I need any media. All documents or photos are either saved to the cloud or my NAS at home (or both, generally). I don't think a laptop is a good place to store any valuable data, too easy to get lost or damaged, so I keep everything that matters elsewhere. This makes 128GB for the internal drive plenty for normal usage.
 
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