While enterprise-level backup options may offer robust solutions for workgroups, Apple’s Time Machine service is a great backup option to use for a single machine since it preserves the OS configuration along with all user data and applications. However, if you have used several Time Machine drives over time, you may find OS X suddenly showing Notification Center warnings that there have been no backups for a given number of days.
This may occur even if you have a Time Machine drive or Time Capsule on-hand that you regularly and successfully use to back up your Mac.
Time Machine supports multiple disks, and when you attach a newly formatted drive OS X may ask whether or not you wish to use the drive for your Mac. You can also manually specify additional Time Machine destinations in the system preferences, including both network storage and local storage. Either way, if you end up with multiple destinations configured, OS X will revolve through them when running its hourly backups to ensure even distribution of your backups.
This means that if you have a working Time Machine drive attached to your system, OS X may successfully back up to it, but then attempt to back up to the other destinations you have configured. At first OS X will simply skip them, but after 10 days you will begin to see the Time Machine warnings appear.
Solutions: Find or remove
The solution to this problem will depend on your particular situation, but boils down to two approaches:
- Find the Disk
Finding and attaching the Time Machine drive that is mentioned in the warning will allow OS X to update the backups on it, and dismiss the warning. You can get more details about the drive by clicking the “Details” button in the warning; however, overall Time Machine will only give you the name of the disk, and often you may end up with non-distinguishable names like “Backup” or “My Backups” or “Time Machine Backups.”
- Remove the drive
If you cannot find your Time Machine drive, or if it has been repurposed or is otherwise unavailable, then your second option is to remove the drive from Time Machine. This approach has the benefit of preventing the warnings from showing, but also will not affect the status of the backups on the drives you remove (ie, they will not be deleted). With this, if you add the drive again, OS X will recognize the backups on it and offer the option to inherit them for the current system. This will allow you to essentially pick up where Time Machine left off with the backups for this drive.