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Use multiple drives with Time Machine System 10.8
If you go to the Time Machine System Preference and click on Select Disk, if you already have a disk set as your backup and you select a second disk, you are presented with a dialog asking if you want to delete the current disk or use both disks for Time Machine. You are told that if you us both disks, Time Machine will take turns backing up to both disks. Nice touch to allow backups at work and home to happen at the same time!

[kirkmc adds: This is one of the features that Apple mentioned about Time Machine in Mountain Lion. The backups rotate, and you can either use disks at home and at work, or even two disks in the same place if you are paranoid about backups (as I am).]
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Use multiple drives with Time Machine | 11 comments | Create New Account
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Use multiple drives with Time Machine
Authored by: Jwink3101 on Aug 16, '12 07:43:50AM

In Lion, you could use multiple drives but you had to specify which drive you wanted. So when I had my laptop at home, I had to set the home external and visa versa at work. This makes it a tad easier since I can just plug it in, but, it doesn't seem all that revolutionary.



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Use multiple drives with Time Machine
Authored by: Tantali on Aug 16, '12 12:39:55PM

I'm assuming this doesn't allow different exclusion-settings for each drive?



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"Time Machine will take turns backing up to both disks"???
Authored by: hamarkus on Aug 16, '12 01:31:37PM

What does this mean if you have both destinations connected? Will TM backup one hour to one of them and the next hour to the other? Or will it back up to both of them in parallel (essentially creating to identical backups assuming they got started at the same point in time).



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Use multiple drives with Time Machine
Authored by: barefootguru on Aug 16, '12 03:50:53PM

Note on 2 of our 3 Macs Time Machine stopped doing any backups after adding a new drive to the pool. A restart fixes it.

hamarkus: the first hourly backup will go to the first drive, the second hour to the second drive, etc. i.e. Time Machine cycles through the drives each time it does an hourly backup.



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How will it thin out the two TM backups?
Authored by: hamarkus on Aug 21, '12 12:03:37PM

Will the daily backups (after 24 h) also be distributed over the two backups, and the weekly backups as well? Or will both backups contain all daily (up to 30 days) and all weekly updates (which would make them close mirrors, not full mirrors as the backups will always be one hour apart)?



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How will it thin out the two TM backups?
Authored by: barefootguru on Aug 21, '12 01:11:58PM

The latter: each volume Time Machine backs up to is treated as its own set. I have two drives which are permanently connected, so have almost identical backups on both of them (within an hour, as you say).



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Use multiple drives with Time Machine
Authored by: lrw on Aug 17, '12 01:39:34PM

I had the work/home backup problem solved in a simple way - I named the disk at work and the disk at home the exact same name. There was no need to change any settings. Everything just backed up nicely relative to when that particular drive was installed. This is on MacOS 10.7.x.



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Use multiple drives with Time Machine
Authored by: kitus on Aug 18, '12 03:06:53AM

I would appreciate you guys clarifying a couple of things. Imagine that I have both my NAS (which features TimeMachine support) and also the USB Disk onto which I also want to backup my Mac. How do I manage both backups? can I create different exclusion profiles? can I backup to the USB disk is available while deferring the backup to the NAS?



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Use multiple drives with Time Machine
Authored by: barefootguru on Aug 21, '12 01:34:24PM

Tantali/kitus: the exclusion list is shared across all backup drives.

The logic is simple: you add multiple drives to the Time Machine prefs, and Time Machine will rotate between all drives which are available. It's automatic when the hourly trigger time is reached: TM will switch to the drive with the oldest backup. If a drive is unavailable TM will choose the next oldest one.

(I'm occasionally still seeing issues when a drive first becomes available—sometimes Mountain Lion needs a reboot for it be used.)

e.g. I have 4 drives, A, B, C, and D. A & B are currently offsite, so one hour TM will back up to C, the next hour will be D. When I plug in A, TM will take 3 hours to cycle between them.

kitus: to exclude the NAS you'd need to make it unreachable, but not sure why you'd want to—2 backups are better than 1.



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Use multiple drives with Time Machine
Authored by: barefootguru on Sep 17, '12 03:04:54PM

Instead of 'volume with oldest backup' the logic appears to be 'volume with oldest *attempted* backup'. Most of the time this is the same, but if a backup fails that volume will miss its turn and have to wait in the cycle again—which makes sense—if an error is preventing a backup to a particular volume you don't want all backups to wait on that volume.



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Use multiple drives with Time Machine
Authored by: barefootguru on Sep 19, '12 01:03:32AM

When restoring using the Star Wars interface, Time Machine will only show backups on the volume most recently backed up to (plus any local backups)—even if more than 1 drive is accessible.



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