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I have another question regarding Time Machine problems (Time Machine always makes a full backup), which I think I have resolved. As soon as I am sure, I will post my solution there.

I am now getting the sort of behaviour I expect from Time Machine (proper incremental backups) but it seems to skip some backups.

I found this morning, that it had not backed up overnight, and I found this afternoon, when I got home from work, that it had backed up this morning, but not during the day.

The iMac was put to sleep, and, obviously, there was no activity to warrant more backups. However, the control panel does say the it keeps “Hourly backups for the past 24 hours”, and that’s clearly not what’s happening.

The question is, is this the correct behaviour of Time Machine: only making backups when the machine has been busy? This would imply that the message is not quite correct.

  • If you acknowledge that there was no new data to backup, then your question is simply nitpicking the ever-so-slightly-inaccurate-but-useful-for-the-vast-majority-of-people wording of "hourly backups for the past 24 hours", isn't it? – tubedogg Aug 21 '17 at 19:56
  • @tubedogg Not really. (a) I believe that in the past TM did make hourly (differential) backups regardless, so this behaviour is different; (b) I’m not sure that not having a backup for the past 12 hours is ever-so-slightly-inaccurate. In any case, I think I now understand the answer. – Manngo Aug 22 '17 at 9:51
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Time Machine backs up only the files that changed since the previous backup, to make future backups much faster, if nothing has changed on the Mac or the computer has been asleep then there's nothing to warrant a backup.

  • For a "running" machine it is not really likely that actually nothing will change in a hour. For example syslog will be written to, as well as other "logs" and these sort of files. – Lacek Jan 27 '18 at 12:29
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I have found this link: FlexiTime Machine: how Sierra changes backups

In it, there is a discussion of how Sierra appears to have changed how the scheduling works. It seems that TM is more discriminating about when to make a backup, and if, for example, there have been no changes since the last backup, then it just won’t bother. The Hourly Backups message is out of date.

The article also goes on to say that this seems to unannounced. It may be speculative, but it is probably correct.

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Time machine does not backup when the computer is asleep.

  • If your machine can Power Nap, then it can take Time Machine backup when asleep using Power Nap. – Bosque Bill Feb 27 '18 at 22:53

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