A backup you can actually restore
A backup that has never been opened is a guess. How to make one, how to test it, and the recovery keys you need before erasing an encrypted disk.
The two reinstall guides on this site both start with the same instruction: back up first, and test the backup. That is not boilerplate. A clean install does not offer a second confirmation once the volume is erased, and the most common irreversible loss is not a failed backup — it is a backup that was made, looked plausible, and turned out to contain nothing useful when somebody finally tried to open it.
This page turns that instruction into a short procedure you can finish in an afternoon. It covers what to copy, where to put it, how to prove the copy works, and the encryption keys that decide whether an otherwise perfect backup is readable.
Decide what has to come back
Start by listing what cannot be recreated. Downloads, applications, the operating system and most game libraries can be fetched again. Photographs, scanned documents, tax records, work files, project source, password-manager exports, browser bookmarks, email stored only locally, device configuration files and licence certificates usually cannot.
Walk the machine rather than trusting your memory of it. Open the Desktop, Documents, Downloads, Pictures and any second drive; check for a folder somebody put at the root of C: or in a shared location; check the Downloads folder of every browser profile, not just the one you use. On a Mac, check the user folder and any external volume that was used as a scratch disk.
- Personal files and folders, including anything on a second internal drive.
- Browser bookmarks and any passwords held only in a browser profile — export from the browser's own settings.
- Application data that is not synced: local mail archives, notes databases, project files, virtual machines, licence files.
- Photos that exist only on the device, not in a cloud library that will still be there after the erase.
- A plain-text list of the applications you will reinstall, so the first day after setup is not spent remembering names.
Where the copy should live
One copy on one external disk is better than nothing, but it is still a single point of failure. The familiar 3-2-1 shape is worth following in a modest form: three copies, on two different kinds of storage, with one somewhere other than the desk the computer sits on. For most home reinstalls that means the original files, a local external drive, and a cloud copy or a second drive kept elsewhere.
On Windows, File History, OneDrive and a plain external drive all work. On a Mac, Time Machine to an external volume is the standard answer, and Apple's instructions are the ones to follow for its exact menu path. A manual copy to an external drive is also fine — the tool matters less than the verification step below.
The wrong place for a backup
A backup on the disk you are about to erase is not a backup. Neither is a second partition on that same physical drive, nor a cloud folder configured to sync deletions — wiping the machine can propagate the wipe.
Prove that it restores, not just that it exists
A backup job reporting 'completed' tells you the software ran. It does not tell you the files are readable, complete, or the version you expected. The test is deliberately mundane: pick three files of different types, open them from the backup device while it is disconnected from any sync process, and confirm the contents.
Choose one large file, one small document you can read a sentence of, and one file with a date you care about. If it is a photo, view it. If it is a document, open it and scroll. If it is an archive, extract a single item. Then confirm the backup device is not still mounted read-write on the machine you are about to erase, and put it somewhere safe.
- Open three files directly from the backup medium — do not copy them back to the original machine first.
- Check file sizes and modification dates against the originals for anything that looks truncated.
- Confirm the backup contains the folders you listed in step one, not just the default library folders.
- Disconnect the drive after verifying. Leaving it attached during Windows setup is how a backup gets selected as an installation target.
- If using a cloud service, confirm the files are fully uploaded and not merely queued, and check that deletion sync will not follow the wipe.
Encryption keys decide whether the backup is readable
Modern Windows devices often ship with BitLocker device encryption already active, and macOS with FileVault. After a clean install, the old encryption key is gone. If the only copy of a file is inside an encrypted volume or an encrypted backup, the recovery key is the only thing that opens it.
Find the BitLocker recovery key before you begin — for many consumer devices it is stored in the Microsoft account used on the machine. On a Mac, note the FileVault recovery key if one was issued, and make sure you know the Apple Account password that Recovery will ask for. Also record any password for an encrypted external backup or a password-manager export.
Do this before booting from USB
Once the volume is erased, a missing recovery key is not recoverable at any price. Verify the key on a second device, not only on the machine you are about to wipe.
The five-minute pre-flight
Immediately before you start the reinstall, run this list once. It is short on purpose; the point is that every item has a definite yes or no answer.
- Three files opened successfully from the backup device.
- Backup device disconnected and stored away from the computer.
- BitLocker or FileVault recovery key written down and accessible on another device.
- Microsoft account or product key, and Apple Account password, confirmed.
- A second device available for looking up instructions while the first is reinstalling.
- Anything that was only in a browser (saved passwords, open tabs, two-factor recovery codes) exported or otherwise accounted for.
Mistakes to avoid
- Trusting a 'backup complete' notification without ever opening a file from the backup.
- Leaving the backup drive plugged in while Windows setup shows the partition screen, then deleting it by mistake.
- Forgetting that BitLocker device encryption was on and not having the recovery key.
- Assuming cloud-synced files are safe without checking that a deletion on the wiped machine will not sync away the cloud copy.
- Backing up only the default library folders and missing a project directory kept on a second drive or at the root of the system volume.
Questions people ask
Is a system image better than copying files?
An image can restore the whole machine quickly, but it also restores whatever problem you were trying to leave behind, and it is harder to inspect. For a clean install, a readable copy of personal files is usually the safer and more portable choice.
How long should I keep the backup after reinstalling?
Keep the external copy for at least a few weeks and until you have opened everything you restored. Do not wipe the backup drive to reuse it on the first evening.
Do I need to back up my applications?
No — reinstall them from their publishers. Do back up their data files and any licence keys, and keep a list of the application names.
What if the only copy is on a drive that is already failing?
Stop using the machine for anything else, copy the most important files first, and do not run a disk repair or a reinstall before the copy is secured. A failing drive can be made unreadable by exactly the operations that seem helpful.
Sources and further reading
- macOS User Guide Apple · Official user guide The first-party reference for backup and encryption prerequisites before an erase.
- Microsoft Support — Windows help Microsoft Support · Official support documentation The first-party reference for activation and update behaviour after a clean install, and for the built-in troubleshooters.
- Download Windows 11 Microsoft · Vendor download page The three options are not interchangeable. We cite this page for which one suits an upgrade, a clean install or a virtual machine, and for the rule that the image comes from the vendor.
Editorial note. This article describes decision logic and checks, not laboratory results. It quotes no benchmark, no test score and no screenshot. Screen labels are reproduced so you can find them on your own device; the sources listed below are the vendor documentation they were checked against.