I remember back my Windows days when I would reinstall Windows regularly. All it would take is a few too many BSOD or perceived slow downs or a new release from Microsoft.Several days ago I bought a 2TB replacement drive for my backup MacBook unibody. When I first installed it I used Carbon Copy Cloner to move the entire contents of my 256GB SSD onto the 2TB HDD. One thing I noticed was that things were really slow. Second thing I noticed was that not everything worked. Did I mention it was slow?Since then I reinstalled OS X Yosemite from USB (no recovery partition on this machine). And now things are humming along nicely. The performance has been restored. I think there are a few explanations.(1) the compression of bits on the drive is higher and although its a 5400 rpm drive the bits are more closely packed and so they are probably faster in and out of the carious stages of the hardware.(2) The drive name changed from the SSD to HDD. That means that certain apps that know the full partition path were failing. Those failures must have escalated into the various parts of the OS thus turning into perceived slowness.The bottom line. OS X needs to be reinstalled to improve perceived performance issues just like Windows. It looks like the distance is narrowing.