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Does an account has to be logged on to trigger the Time Machine backup, or does Time Machine run without any session active?

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An interesting question that I hadn't thought of before. Now you have mme wondering too.... – Megan Walker May 21 '13 at 8:57
I have no answer, but it wouldn't be strange not to backup when no user changes its files. – mouviciel Jun 4 '13 at 7:34
@mouviciel But Time Machine runs hourly, so I think this scenario is possible: user is logged in; at 1000, Time Machine runs; at 1010 user makes some edits, then logs out. At 1100, no-one is logged in: will Time Machine start and back up those recent edits? (NB I've simplified: the times won't be on the hour, but I do believe it runs the backups every 60 minutes.) – Ashley Jun 6 '13 at 12:42
@Ashley - I didn't think of that scenario. Good point. – mouviciel Jun 6 '13 at 15:20

Time Machine is a system-level daemon, not a user-level process. Time Machine will work even if you are not logged into your Mac.

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Please provide verifiable evidence. – smokris May 23 '13 at 19:15
@smokris Time machine's process is called backupd.It is started by root and is a son of launchd. So it is a daemon. I also think the "d" stands for daemon. – Rnhmjoj May 25 '13 at 22:49
I really don't see it happening. I've several users on my mac, not all with admin rights. I don't know if it's important. – Rabskatran May 27 '13 at 9:09
@smokris - Launch Time Machine after a period with no user logged on, then check the dates of the last backups. This will give a definitive answer, be it yes or no. – mouviciel Jun 4 '13 at 7:33
After seeing bmike's answer below, I now must backtrack a bit to clarify that I use Time Machine Server with Time Machine. While the process is system-based as I said above, it is quite possible that an unmounted backup hard drive would fail. – bispymusic Jul 5 '13 at 15:34

No. Mac OS X doesn't mount external drives so even if backupd was told to back up from an ssh session using tmutil startbackup the external drive that Time Machine normally uses would be absent and the backup would fail.

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