I have reinstalled my Mac OS (10.9) and had to specify again my external hard drive for backups in Time Machine preferences. Now i can enter Time Machine and see my previous backups.

However, Time Machine preferences show

Oldest backup: None
Latest backup: None

In particular, the estimated size for a new backup indicates that it will have to make a complete backup, without taking into account that most files in my home directory have not changed since the previous backup (before the Mac OS reinstallation).

Can i configure Time Machine to recognize the previous backups and to only backup modified files in my home directory?


Yes. The command line utility can assist

man tmutil

You'll want to use the verbs inheritbackup, setdestination and/or associatedisk to point out the pre-existing backup you wish to adopt by the new Mac/OS.

This is covered in very nice detail on the help / manual page - pay attention to the section after this sentence:

A concrete example of when and how you would use associatedisk:

  • 2
    Thank you, but so far it didn't work. Instead of setdestination, i used the Time Machine pane in System Preferences and i used the verbs inheritbackup and associatedisk, but it still does not recognize previous backups. I will try to experiment with it later. – Alexey Sep 13 '15 at 20:06
  • I also tried to use Migration Assistant as indicated here, but it didn't even find the backup disk, maybe because i have already fiddled with it... – Alexey Sep 19 '15 at 9:58
  • After trying different commands with tmutil without visible effect, i started a backup. The backup turned out to be about 5 times smaller than a complete backup would be, but also about 5 times larger than a normal incremental backup i would expect, and it backed up some files i didn't touch (i checked this with BackupLoupe). As a result, Time Machine also started to show correct Oldest and Latest backup times. This all is quite confusing. – Alexey Sep 19 '15 at 13:07
  • @Alexey It sounds odd, but I bet the next few intervals are sane. Also - BackupLoupe is so good for figuring out things, but I've also had times when its monitoring affected the backups - so try turning off the loupe scanner if you have that enabled. – bmike Sep 19 '15 at 14:41

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