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I've downloaded the OS X El Capitan Installer, but when I try to put it into action, the machine refuses access to my MacIntosh HD because it says Time Machine is using the drive for backups. How do I get Time Machine out of the way so I can upgrade the OS?

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If you're sure you don't need backups, this could be a three step process:

  1. Turn off Time Machine
  2. Delete the folder Macintosh HD/Backups.backupdb after making sure no other machine data will be lost.

If that works, great. If not, open terminal and type

  1. sudo tmutil disablelocal

That might take several minutes to clear up the local storage cache of backup files. After maybe 15 minutes of no activity, reboot and try the installer again. If you have a specific error message - look in the installer logs: /var/log/installer.log

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I've never known Time Machine to hold up an OS update, however…

System Prefs > Time Machine [or from the menu bar if you have set it to show there] you can tell Time Machine to halt its current backup, or switch it off completely.

TM pic

A late note: if you have local snapshots enabled [which I don't, in the picture above] Time Machine changes its "Time Machine keeps:" line to
"Time Machine keeps local snapshots as space permits, and:"

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    This is clearly not something most people end up with. If you use Server.app to enable other Macs to use Time Machine, it doesn't place a blocking store of backups on the filesystem. About the only way I can see this is booting in target disk mode and enabling another Mac to backup or perhaps cloning a drive that had backups and then commingling it with a bootable system. – bmike Nov 2 '15 at 18:37
  • I'm a layperson but my guess is that Time Machine interfered with the installation process because Macintosh HD had been selected as the Time Machine drive. Part of my workaround was to deselect it and replace it in Time Machine with an external HD. – petrenyc Nov 3 '15 at 2:30
  • Ah, yes - that would do it. I'm not sure how it even let you select the boot volume as a Time Machine backup. It is certainly a more valuable resource on a separate physical drive. – Tetsujin Nov 3 '15 at 7:30
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Thank you -- I succeeded following your first suggestion: 1) trash backups.backupdb 2) turn off Time Machine

Once the install was complete I copied backups.backupdb from the trash to Macintosh HD. Dunno if I can access it ... Time Machine is no longer offering Macintosh HD as a backup drive candidate.

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