You plugged your iPhone in a few times over the years to “back it up,” and macOS has quietly archived a full copy of each device state — every time. A modern iPhone backup is 20–50 GB; a Mac that’s imported two iPhones over three years can be eating 100+ GB of ~/Library/Application Support/MobileSync with zero obvious sign.

Here’s how to find, delete, and stop them.

Check what backups exist

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

That lists every iOS backup by folder + size. A single BU-<hash>/ folder = one device state. Multiple folders = up to one per device (plus failed partials).

ls -lt ~/Library/'Application Support'/'MobileSync'/Backup

The sorted-by-date view shows which are ancient.

Safe to delete? Which one do you keep?

You should keep your current, working backup (the one you’d use to restore if your phone dies). The rest is dead weight.

Old devices (phone you traded in 3 years ago)? Delete.
Old partials/failed backups? Delete.
Your active iPhone’s latest backup? Keep.

How to tell which is current: sort by date; the newest BU-* is the keeper (and check Settings → General → iPhone Storage → Backups → “This Mac” in iOS for the true record).

Delete safely (2 ways)

The clean way — Finder

Connect the iPhone to the Mac.
Finder → sidebar → your iPhone → General → Manage Backups.
Select the old backups in the list → Delete.

The terminal way (the exact folders)

# view sizes first
du -sh ~/Library/'Application Support'/'MobileSync'/Backup/* 2>/dev/null

# remove ONE old backup by its folder name (or date)
rm -rf ~/Library/'Application Support'/'MobileSync'/Backup/BU-1234abcd

⚠️ Only the folder you name; never rm -rf the whole Backup directory — that’s every backup at once.

Safe to delete? Yes — for old devices/partials. The delete is permanent (backups aren’t in the Trash until you move them manually), so be deliberate.

How backups keep coming — and how to stop it

“Back Up Now” creates on demand → disable if you prefer iCloud backup (iOS Settings → Your Name → iCloud → iCloud Backup → Off) and rely on that instead.
Auto-backup on sync: uncheck “Automatically back up your iPhone” in Finder’s sync panel.
Encrypted backups are bigger (they store passwords/health) but also don’t appear in du size as expected — treat them the same; you can check “Encrypt local backup” status in Finder → iPhone → General.

If you switch fully to iCloud backups, you can delete all local MobileSync copies — your phone’s health, passwords and data live in iCloud instead.

The folder these hide in (same one everything hides in)

~/Library/Application Support/MobileSync is a great example of the general “uninstall ≠ gone” problem — and it shares a wall with every other leftover family discussed in our app leftovers guide and the ~/Library caches explainer.

See all backups at once

A full byte-accurate view of ~/Library/Application Support/MobileSync (and every other heavyweight folder) is a disk map problem — exactly the thing a scanner is built for. Rank by bytes, see the 40GB “BU-…” blocks, choose which to keep, and stage to the Trash.

DiskInsight shows hidden Application Support & Containers paths, ranks by real size, and stages anything you pick to the macOS Trash for review — reversible, clearly labeled, no surprise deletions. 100% local, no telemetry, one-time price.

TL;DR

du -sh ~/Library/'Application Support'/'MobileSync'/Backup/* — spot the GBs.
Keep the newest backup of your current device; delete the rest via Finder → Manage Backups (or rm -rf the exact BU-* folder).
Prefer iCloud backups → then delete ALL local copies.
Uncheck auto-backup so it stops regenerating.

Old iPhone backups are the most polite form of “your Mac is full”: easy to see, easy to remove, and definitely not worth space you need.

Get DiskInsight Pro · $9.99 one-time to find every heavy folder — not just backups. Or start with the Mac cleanup guide.

Old iPhone Backups Are Eating Your Mac's Storage - How to Delete Them