If you've ever connected an iPhone or iPad to your Mac and backed it up locally rather than to iCloud, that backup is sitting on your disk right now under ~/Library/Application Support/MobileSync/Backup/, in a folder named after the device's unique identifier rather than something recognisable like "Deepak's iPhone". Modern macOS does this through Finder — which took over from iTunes for device management — and it is easy to forget these backups exist at all, especially from a phone you upgraded or sold two devices ago. Each one can run from a few gigabytes to well over 15 GB depending on how much photo and app data was on the device at the time, and because there's no reminder that they're there, they are one of the more common multi-gigabyte space sinks on a longtime Mac user's drive.
Where backups actually live
Finder stores each local backup under ~/Library/Application Support/MobileSync/Backup/, with one subfolder per device. The subfolder name is a long alphanumeric device identifier rather than the device's given name, which is part of why these are easy to lose track of — there's no obvious label sitting in Finder telling you "this one is the iPhone 12 I sold in 2023". If you've backed up more than one device to the same Mac over the years, or backed the same device up multiple times without deleting the older copy, you can end up with several of these folders without realising it.
What's actually inside a backup
A backup captures a snapshot of the device: app data, settings, message history, photos and videos not already safely in iCloud, and more. The size varies enormously based on what's on the phone — a device with a large photo library and many apps with substantial local data will produce a much larger backup than a lightly used one. As a rough sense of scale, backups commonly range from a few gigabytes up to 15 GB or more.
There's an important difference between encrypted and unencrypted local backups: an encrypted backup additionally includes categories of data that an unencrypted one skips entirely, such as saved passwords and Health app data. If you've ever turned on "Encrypt local backup" for a device, that backup is doing more than a plain one, which is also part of why encrypted backups tend to run larger.
Why these pile up unnoticed
The pattern is familiar to anyone who has owned more than one iPhone: you back up the old phone before setting up the new one, migrate everything over via the new device's own setup process, and then never think about the old backup again. It sits in MobileSync/Backup indefinitely, keeping several gigabytes reserved for a device you no longer own and, in most cases, will never need to restore from again.
It's also common to have more than one backup of the same device if you've run a manual backup multiple times over the years without ever deleting the previous one first. Unlike iCloud backups, which typically keep only the most recent snapshot per device, local backups made through Finder can accumulate side by side unless you actively manage them, so a Mac that's been used to back up two or three devices over several years can easily be holding five or six of these folders at once, most of them long past being useful.
Deleting a backup the supported way
macOS gives you a supported way to see and remove these backups from within Finder rather than reaching into the MobileSync/Backup folder and deleting a subfolder by hand. With a device connected, select it in Finder's sidebar and look for a backup management option — depending on your macOS version this may appear as Manage Backups in the device's summary pane, or under a General or Backups section of the device settings shown there. The exact wording and location have shifted slightly across macOS releases, so if you don't see it in one place, check the other — the underlying feature is the same: a list of backups stored on your Mac, each with its date and approximate size, and a way to delete the ones you select.
This in-Finder route is worth using instead of manually deleting a folder from MobileSync/Backup, because it's the path macOS expects and keeps its own bookkeeping about existing backups consistent with what's actually on disk.
If you'd rather confirm sizes yourself before deciding what to remove, you can check the backup folder directly from Terminal:
du -sh ~/Library/Application\ Support/MobileSync/Backup/*
That lists each device's backup folder with its total size, which is a useful sanity check before you go into Finder to delete anything — you'll know in advance which one is worth reclaiming.
Deletion is permanent
There is no undo once a backup is deleted, and no way to recreate it without the original device connected and willing to back up again. If the phone or iPad the backup came from has since been wiped, sold, or handed to someone else, that backup is the only remaining copy of whatever it contains — old message threads, camera roll items that were never in iCloud, app data from apps you've since deleted. It's worth a moment of thought before deleting: is this backup from a device you still have, or definitely don't need to restore from? Once you're sure, deleting it through Finder's backup manager is safe and immediate, and the disk space is freed as soon as it completes.
Conclusion / VolumeLens tie-in
Local iPhone and iPad backups are a good example of storage that's completely legitimate, genuinely useful while you need it, and easy to forget the moment you don't. The fix isn't complicated — Finder already has the tools to list and remove them — but it does require knowing to look, since nothing on your Mac proactively tells you these folders exist or how old they are.
That's the gap we built VolumeLens to close. We wanted a way to actually see where a Mac's space had gone — including folders like MobileSync/Backup that sit quietly in your Library — without uploading anything, without a background service, and without needing an admin password to look. You can download VolumeLens for free, point it at your home folder, and see exactly how much space old backups, caches, and forgotten project files are holding onto, then decide for yourself what's worth keeping.