The overwhelming majority of us using Time Machine do so very simply: it backs up from a single volume, the startup disk, to a single backup destination on another volume.
Time Machine is actually capable of far more than just that. This article explains what else it can do, and how it does it.
By default, Time Machine will configure itself to back up all internal drives. As relatively few Macs now have more than one internal drive, this affects a reducing number of users. Fusion Drives, so long as they are operated as such, are considered to be single logical volumes, so are not shown separately.
Also by default, Time Machine will not back up external drives, which are more likely to come and go with time. The way that you go about setting this works a little strangely: with the drive connected and mounted, click on the Options… button in the Time Machine pane. You should see the external drive listed in your exclusions. You can then select it and click on the – button below. You remove it from the exclusions in order to add it to the list to back up.
In general, you should be very cautious about adding external drives as backup sources, and only do so if you can be certain that they will be mounted during each backup. For most with external drives, a different method of backing up is more appropriate.
Few users have more than one volume which they can use for the storage of Time Machine backups. However, if you use a laptop in two or more locations and have facilities to back up at each of them, this can make very good sense. It is worth serious consideration if you use your laptop at home and your office, or in two or more different offices, for example.
Time Machine manages multiple backup destinations in a simple and flexible way which you need to understand before setting this up. Each backup destination will contain a complete Time Machine backup, but automatic backups are made to only one of those destinations each hour, in rotation.
Let’s say that your MacBook Pro is used in three locations, each with its own destination volume, named Vol1, Vol2, and Vol3.
On the first hourly backup, Time Machine looks first for Vol1. If it finds it mounted, then it will run that backup to Vol1. At the next hourly backup, it rotates backup destination, and looks for Vol2. If it finds it mounted, then it runs the automatic backup to Vol2. If it cannot find Vol2, it looks for Vol3, and will back up to that instead. If it cannot find Vol3, then it will make that backup to Vol1 again.
There are several snags with this scheme. The first backup made to each of the backup destinations will necessarily be complete, and very large. With three rotating backup destinations, that means making three full backups before you even start making routine hourly ones.
Each time that you mount one of your backup volumes, the very next backup to it will be hefty, as it will consist of all the files that have changed since your Mac was last backed up to that volume. If you have been working away for several hours or days, that could be another large backup load.
To add another destination volume for your Time Machine backups, ensure that volume is mounted, then click on Select Disk in the Time Machine pane, and add the volume to the list of Backup Disks.
Once Time Machine is backing up to two or more destination volumes, you can adjust which it uses by manipulating its queueing system. For example, if it is in the midst of making a very long backup to one volume and you want it to pause that to perform a shorter backup to another one, open the Time Machine pane and click the x next to the progress bar of the current backup.
When it comes to perform its next automatic backup, it will then look for the next volume in the queue, and try to back up to that. Once that is complete, you can resume the long backup using the Time Machine pane again. However, you cannot alter the order of volumes in the queue: if you want it to skip backing up to one volume, you will need to cancel that backup or unmount that backup volume.
Some recommend backing up to two or more volumes using different techniques, such as scheduling one using a third-party Time Machine utility or a LaunchAgents property list. I’m not convinced that is any better, and it loses some of the advantages of the standard system. If you want to use two backup systems, then it might be better to use two different software systems too.
Backing up to two or more destinations can become complex. When there are problems, working out what is wrong can be difficult, and you will probably need to browse your log using Consolation (free from Downloads above) or similar.
The Time Machine Mechanic (T2M2, also from Downloads) should provide a detailed analysis to help you get to the bottom of advanced backup schemes. Version 0.3b1 does now cater for multiple backup destinations, manual backups, and more.