Every time you plug an iPhone into a Mac and press "Back Up Now" in Finder, a full device backup lands on the internal disk. After a few years and a couple of phones, MobileSync/Backup routinely holds 40-120 GB — and it counts toward the opaque "System Data" bucket.

1. See what you actually have

du -sh ~/Library/Application\ Support/MobileSync/Backup/*

Each hex-named folder is one backup of one device. Multiple folders for the same iPhone are normal if you ever restored or switched iTunes/library locations.

2. Delete old backups safely

The clean way is inside Finder: connect the phone → General → Manage Backups → select an old backup → Delete. This keeps the newest one and removes Finder's bookkeeping along with the files.

To delete a specific folder directly (e.g. a backup of a phone you no longer own), first confirm which device it belongs to:

plistutil -p ~/Library/Application\ Support/MobileSync/Backup//Info.plist | grep -E 'Device Name|Last Backup' 2>/dev/null \
  || defaults read ~/Library/Application\ Support/MobileSync/Backup//Info.plist "Device Name"

Then remove that folder only. Never delete the whole Backup directory if any device still syncs.

3. Move future backups to an external drive (the permanent fix)

# quit Finder's device window first
mkdir -p "/Volumes/BackupDrive/iOS-Backup"
cp -R ~/Library/Application\ Support/MobileSync/Backup "/Volumes/BackupDrive/iOS-Backup/"
mv ~/Library/Application\ Support/MobileSync/Backup ~/Library/Application\ Support/MobileSync/Backup-old
ln -s "/Volumes/BackupDrive/iOS-Backup/Backup" ~/Library/Application\ Support/MobileSync/Backup

Future Finder backups follow the symlink to the external drive. Keep the drive plugged in during backups. Once verified, delete Backup-old and reclaim the space.

4. iCloud-only alternative

If your iCloud plan covers the phone, switch to iCloud backups (Settings on iPhone → your name → iCloud → iCloud Backup) and delete local ones entirely — zero Mac disk used, at the cost of trusting the cloud and a subscription.

Related

iOS backups are one of the classic hidden eaters; the rest are mapped in System Data explained and the cleanup hub. After deleting 40+ GB, treat yourself to a visual check with DaisyDisk — few things are as satisfying as watching the sunburst shrink.