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Running Yosemite with a half terabyte HDD. Laptop sometimes hangs, apparently not heat related. I have tried repairing file permissions to no effect. Tried Repair Disk but no errors found. What Disk Utility did find was that if I highlight the main or root hdd and click Info, it says it has 20 MB free space. If I highlight the partition in it, Info says there’s 200 GB free space. So obviously there’s a foulup in the reporting between the two. Maybe this has something to do with the intermittent hanging. How do I fix this?

This model is renowned for the hard drive I/R cable failures. Apple was even replacing them for free for a while. Try changing out with this one: MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement