My Time Machine backup drive is full

Even if your Time Machine drive matches the capacity you’re backing up, it’ll eventually fill up with copies of files that have changed. It’ll then start deleting the oldest versions.

To avoid this, you can switch to backing up to a bigger drive – but do it the right way. If you select a new disk in Time Machine’s preferences, you’re asked whether to use this instead of or in addition to the current one. The latter backs up to both drives in turn (useful if you back up at two locations), starting now, maintaining two backups, but still deleting those old files to fit.

The former uncouples the old drive from Time Machine and uses the new one; older file versions won’t appear in Time Machine unless you hold Opt, click its menu bar icon, and pick Browse Other Backup Disks. To migrate old backups to the new drive, see bit.ly/newtmdrv.