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?

1 Answer 1


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
    Commented Sep 13, 2015 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
    Commented Sep 19, 2015 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
    Commented Sep 19, 2015 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
    Commented Sep 19, 2015 at 14:41

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