Your Mac does not have enough space to back up your iPhone: four ways around it

Finder starts backing up your iPhone, then stops: the Mac does not have enough free space. A local backup can be anywhere from 5 GB to over 100 GB, depending on photos and apps on the phone.

1. Delete old backups first

The Mac may be full of backups from phones you no longer own. In Finder, select your iPhone, click Manage Backups, and delete any you do not need. Or check sizes in Terminal:

du -sh ~/Library/Application\ Support/MobileSync/Backup/* 2>/dev/null

Terminal needs Full Disk Access to read that folder. iPhone backups taking up space explains which backup is which.

2. Back up to iCloud instead

On the iPhone, Settings, your name, iCloud, iCloud Backup. iCloud backups do not include photos already in iCloud Photos, so they are often much smaller than a Mac backup. You need enough iCloud storage.

3. Make the backup smaller

Most of a local backup is photos and app data. If iCloud Photos is on, photos are not included in the local backup either. Check large apps on the phone in Settings, General, iPhone Storage.

4. Back up to an external drive

Finder has no setting for this, but it can be done by moving the backup folder to the drive and leaving a link in its place. Quit Finder's backup first.

# Move the backup folder to the drive, then link to it
mv ~/Library/Application\ Support/MobileSync/Backup /Volumes/YourDrive/iPhoneBackup
ln -s /Volumes/YourDrive/iPhoneBackup ~/Library/Application\ Support/MobileSync/Backup

If the backup is to make room on the phone before an update, see clean a Mac before selling for the related steps when you are replacing devices.