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I am interested in understanding how much of my 7200 rpm hard drive's capacity is being used, because sometimes performance is very slow and choppy.

The Disk tab in Activity Monitor gives information about Data read/sec and Data written/sec – but is there a way to display such information relative to what it's capable of, e.g. a percentage measure.

Maybe there is a separate utility that could assist with this? Or perhaps, for technical reasons, this is not the way you measure drive performance?

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    What size and how full is the HDD? It also could be highly fragmented depending on how much is being stored and how it's been used in the past. Yes, rotational hard disks do get fragmented under OS X and it can and does have an effect on performance. See iDefrag for more details. You can download it and run it first to see how fragmented the drive is and whether or not you want to purchase it after seeing the results. Note that I'n not affiliated with Coriolis Systems Limited. – user3439894 Sep 3 '16 at 14:05
  • @user3439894 Thanks for your comment – the OS X partition is 1.83 TB and is 17% free (~312 GB). I thought OS X did defragmentation on its own, but perhaps utilities are needed anyway? (Note: It's a 2TB drive with 0.17 TB Boot Camp Partition). – Winterflags Sep 3 '16 at 14:09
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You're reading exactly what you want. A percentage throughput would not give you more information, just less accurate information as to the state of your I/O. Your drive should be pushing about 150MB/s at the most (Spinny disks aren't too fast in general, and worse when mostly used).

I am interested in helping with your main issue of slow and choppy HDD performance. If this is a MacBook or MacBook Pro unibody non-retina, you may have a degraded SATA cable. My first one went out after 2.5yrs in a 2012 MBP. A friends' went bad after five years in her 2010 MBP. It's very common across the support forums too. If you feel data access has gotten slower, or you've started having issues with freezing, spinning beach balls, and/or crashes and reboots, swap the cable before the drive, or both together. Not changing the cable led to my frustration over the slowness of two day shipping twice too often now.

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  • Gelliot, thanks for your answer. I am seeing those symptoms on my non-Retina iMac mid-2011 – perhaps it could be a cable issue there as well? It started performing sluggishly about a year ago. – Winterflags Sep 3 '16 at 13:32
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    Take it to an apple store and ask them to check the drive health if possible. They should do it free and report whether the drive meets Apple spec or not. If not, the cable is more likely to cause problems than the drive itself even at 5yrs. If you can't get to an Apple Store, you really don't have much of an option other than attempting a replacement. Diagnostics requires either a reference for the minimum acceptable speeds or one known good cable to determine whether the first one is bad. – gelliott181 Sep 3 '16 at 13:41

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