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Use multiple backup disks
For extra security and convenience, you can back up to multiple disks. If you work in two locations, you may want to keep backup disks in both places.
Time Machine rotates the backup schedule among the disks. For each disk, Time Machine backs up everything that’s changed since the last time that disk was used. If you designated items that shouldn’t be backed up, the list of excluded items applies to all backup disks.
If you use multiple backup disks, when you click the Time Machine icon 
in the menu bar and choose Enter Time Machine, you see the timeline for all the accessible disks used for backing up. To see the timeline for a specific disk, click the Time Machine icon , press the Option key, then choose Browse Other Backup Disks.