If the Time Machine backup disk for your Mac is full
Your first Time Machine backup includes everything on your Mac. After that, Time Machine finds and saves only new and changed items, so the backups become smaller. Also, as your backup disk fills up, Time Machine deletes older backups to make room for new ones. You may be able to use Time Machine for a long time before running out of space.
If you do run out of space, it’s best to connect a new backup disk. After you connect the new disk, open Time Machine preferences, then select it as your Time Machine backup disk.
To save space on your backup disk, you can choose items to exclude from backups.
On your Mac, choose Apple menu > System Preferences, then click Time Machine .
Click Options, then do one of the following:
To add an item, click the Add button , then select one or more items to add to the list of excluded items. You can also drag items to the list.
To remove an item, select it in the list, then click the Remove button .