Connect a new backup disk to your Mac

When you connect a new disk to your Mac, you’re asked if you want to use Time Machine to back up onto it.

Connect a new disk to your Mac, then choose one of the following in the dialogue that appears:

Set Up: Time Machine uses this disk as a backup disk.
Close: Time Machine closes and the disk is mounted as a regular disk.

If your Mac is in sleep or your backup disk isn’t available when it’s time for a scheduled backup, the backup isn’t performed. Backups resume after the Mac and disk are available again.

When you use Time Machine, Time Machine also saves a local snapshot you can use to recover previous versions of files, even if your backup disk is not attached. These snapshots are created hourly, stored on the same disk as the original files and saved for up to 24 hours or until space is needed on the disk. Local snapshots are only created on disks using Apple File System (APFS).

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macOS User Guide

Connect a new backup disk to your Mac