Connect a new backup disk to your Mac
When you connect your Mac to a new disk, you’re asked if you want to use the disk to back up your files, unless you already have Time Machine backups set up.
Connect a new disk to your Mac, then choose one of the following in the dialogue that appears:
Use as Backup Disk: Your first backup could take a while, but you can use your computer during backups. Time Machine works best if you use your backup disk only for backups.
Encrypt Backup Disk: If you back up to an external disk and don’t encrypt the backup disk, any person who gains possession of the disk can read the information on it.
Decide Later: Time Machine displays the message again each time you connect to the disk.
Don’t Use: Time Machine won’t ask again when you connect to this 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.
On a Mac using Apple File System (APFS), in addition to saving backups on your backup disk, Time Machine saves hourly local snapshots of files and stores them on your computer’s internal disk. If your backup disk isn’t connected, Time Machine continues saving local snapshots on your internal disk, then resumes backing up to your backup disk when you reconnect it.