You go to install or reinstall macOS and it stops cold: "An error occurred while preparing the installation," "This copy of the Install macOS application is damaged," or a download bar that hangs at zero and never moves. The good news is the disk and the Mac are almost always fine — these are download, date, and free-space problems, and each one has a specific fix. Work down this page in order.
First: which failure are you seeing?
macOS installer trouble comes in a few flavors, and the fix depends on which one you hit:
- The download never finishes — the installer sits at "Downloading…" for hours, or the App Store shows the full installer but nothing happens when you open it.
- "This copy of the Install macOS ‹version› application is damaged and can’t be used to install macOS." — almost always a wrong system clock, not a real corrupt file.
- "An error occurred while preparing the installation. Try running this application again." — usually a date/certificate or a partial download.
- "macOS could not be installed on your computer" / "The installation could not be completed" — typically not enough free space or a disk that needs First Aid.
- "Not enough free space" — the installer needs far more room than the final size suggests.
Fix 1 — Set the date and time automatically (fixes most "damaged" errors)
The single most common cause of "This copy… is damaged" and "an error occurred while preparing the installation" is a wrong system clock. macOS checks the installer’s security certificate against today’s date; if the clock is off, the valid certificate looks "expired" and the installer is rejected as damaged.
If macOS still boots: open System Settings › General › Date & Time and turn on Set time and time zone automatically. Then try the installer again.
If you’re stuck in Recovery or a bootable installer (no desktop to open Settings), open Utilities › Terminal and set the date manually with date in [mm][dd]HH][MM][yy] order — for example date 0801120026 for Aug 1, 12:00, 2026. Close Terminal and re-run the installer. If your Mac has no internet in Recovery, this manual step is what unblocks it.
Fix 2 — Free up space (the installer needs more than you think)
A macOS installer reports a small download size but needs roughly 25–40 GB of free space to stage and apply the update. If your startup disk is nearly full you’ll get "not enough free space" or a silent failure.
- Check free space in System Settings › General › Storage.
- Empty the Trash, clear the Downloads folder, and offload big files. Our guide on why your Downloads folder keeps filling up clears space fast.
- Aim for at least 40 GB free before retrying a major-version install.
Fix 3 — Re-download the full installer cleanly
A partial or interrupted download will fail every time you open it. Delete the broken copy from your Applications folder (it’s named Install macOS ‹version›), then download a fresh one:
- From the App Store — search the macOS version by name (Sonoma, Sequoia, Tahoe) and click Get; the full installer downloads to Applications.
- On a slow or dropping connection, use a wired Ethernet adapter if you have one — large installers time out on flaky Wi-Fi.
- Let it finish completely before opening it. If the App Store download itself stalls, sign out and back into the App Store, or restart and try again during off-peak hours (Apple’s servers throttle during launch-day rushes).
Fix 4 — Repair the disk with First Aid, then reinstall from Recovery
If the download is fine but the install "could not be completed," the target disk may have directory errors. Boot into macOS Recovery, open Disk Utility, run First Aid on your startup volume, then choose Reinstall macOS. Reinstalling from Recovery downloads a fresh copy from Apple and keeps your files.
If Recovery itself keeps looping, that’s a separate problem — see Mac keeps booting into Recovery Mode.
Fix 5 — Build a bootable USB installer (when nothing else downloads)
If the Mac can’t download macOS at all — no internet in Recovery, or the App Store keeps failing — make the installer on another working Mac and carry it over on a USB drive. Full steps are in how to create a bootable macOS USB installer. This also lets you do a clean install without relying on Apple’s servers at install time.
Just want the update to install normally rather than a full reinstall? Start with how to update macOS and its "update stuck or failed" checklist. Need to go the other direction? See how to downgrade macOS.
Frequently asked questions
Why does my Mac say the Install macOS app is damaged when I just downloaded it?
Nine times out of ten the file is fine and your Mac’s clock is wrong. macOS validates the installer’s certificate against the current date, so a battery-drained clock or a manually-set wrong date makes a perfectly good installer look "damaged." Set the date and time automatically (Fix 1) and it usually installs on the next try.
How much free space do I need to install macOS?
Plan on about 25–40 GB free, even though the download looks smaller. The installer needs room to stage the new system alongside the old one before switching over. Below ~20 GB free, installs frequently fail.
The macOS download is stuck at zero — is my Mac broken?
Rarely. A stuck download is almost always a network or server issue: flaky Wi-Fi, a VPN or content filter, or Apple’s servers being hammered right after a new release. Try a wired connection, disable any VPN, and retry off-peak. If it still won’t move, download the installer on another Mac and use a bootable USB installer.
Will reinstalling macOS erase my files?
No. Choosing "Reinstall macOS" in Recovery reinstalls the system over the top and leaves your files, apps, and settings in place. Only "Erase" in Disk Utility wipes the drive. Back up first anyway.
My Mac is too old to download the macOS I want. Now what?
Apple only offers installers your model is eligible for. Check what macOS version your Mac can run first. If your Mac is stuck several versions behind and it’s slowing you down, upgrading is often cheaper than you’d expect — a refurbished M1 MacBook Air ($450) runs the latest macOS smoothly, and our Apple clearance section has current-macOS Macs at the lowest prices we carry, from the M2 Air ($549) to the Mac Mini M2 ($599).
Fixed it — or ready for a Mac that just works?
If the installer errors trace back to a failing drive or a Mac too old to stay current, it may be time to move on. We buy working and broken Macs for real store credit — get a trade-in quote — and stock refurbished, current-macOS Macs starting at $450. Browse Apple clearance or see all guides.