I want to defragment my hard drive

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A welcome side-effect of cloning one volume to another is that the files on the resulting volume are largely defragmented. While fragmentation is not as significant of an issue as it used to be (e.g. in the Mac OS 9 days), people that have begun to fill the last 10-15% of their boot volume may see some performance benefit from defragmentation. If you find yourself in this situation, this is also a really good time to consider migrating to a larger hard drive altogether, or to an SSD, which is not affected by fragmentation.

Defragmentation is a natural result of backing up your data to an empty backup volume. Simply prepare your backup volume for use with Carbon Copy Cloner, then use CCC to clone your source volume to your destination volume.

"Clone, wipe, restore" — think twice before you wipe that original volume

It may be really tempting to do the following:

  1. Clone your boot volume — the one with your lifetime of irreplaceable data — to another hard drive
  2. Boot your Mac from that cloned volume
  3. Use Disk Utility to wipe the original volume
  4. Restore the cloned volume to the original volume

Very quickly you'll be booted back up from your boot volume and you'll have a backup to boot, right? In most cases, this would work out great for you, and you'd be fine. There are two really good reasons, however, to stop after the second step and take a breather:

  1. As soon as you erase the original volume, you're down to one copy of your data — you have no backup. The restore task will stress both the source and target disks with massive reads and writes. If either disk were on the verge of failure, this level of stress could push it over.
  2. You really should take the time to verify your backup. I trust CCC with my data, but do I trust that I asked it to copy the right items? Did my source volume have a filesystem problem that went unnoticed?

The Best Practice for defragmenting your hard drive

  1. Establish a backup regimen to a primary backup volume. Test your backups regularly.
  2. Quit open applications and use CCC to update your backup one last time.
  3. Use CCC to clone your hard drive to some physical volume other than your primary backup.
  4. Boot from the cloned volume.
  5. Use Disk Utility to reformat your original volume.
  6. Use CCC to restore your cloned volume back to the original volume.
  7. Boot from the original volume.

Performance upon first boot from a cloned volume will always be slightly slower than normal as Spotlight reindexes your data. When the system has "settled down", you will be able to evaluate whether the defragmentation has offered any performance benefit.