Connect a new backup disk to your Mac
If you connect a new disk to your Mac before setting up Time Machine, you’re asked if you want to use the disk as a Time Machine backup disk. You can also set up a backup disk through System Settings.
Connect a new disk to your Mac. If prompted, choose Allow in the window that appears.
Do one of the following:
Hold your pointer over the dialogue that appears, click Options, then choose Set Up to use this disk as a backup disk with Time Machine. (If you choose Close, Time Machine closes and the disk connects as a regular disk.)
Choose Apple menu > System Settings > General, then click Time Machine. Click Add Backup Disk, select the backup disk, then click Set Up Disk.
Note: A window appears if your disk supports space limits and encryption for backups. Set these options, then click Done.
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).