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'Other' Is Eating Up My rMBP Hard Drive

Use the DaisyDisk program, it will tell you exactly what space you are using for what. It summarises everything for you, then you can keep drilling down until everything is understood.
 
Been using my rMBP for a few hours now and it seems to run a lot snappier, thanks to the fresh install. My re-occuring problems of accessing my Time Capsule drive have also been resolved...no more waiting forever for the Time Capsule drive to appear as an available drive, no more 'drive is not available' messages either.

I have always upgraded from a previous OS's thinking that it can do no harm. I am now a believer of fresh installations!
 
Your problem's fix is pretty simple.

Just boot into the Recovery HD and repair the Macintosh HD. That fixes your phantom loss of disk space. It happens with time machine and if you delete a lot of files a lot.
 
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