The dangerous part of preparing bootable media is not the writing. It is the selection. Every tool asks you to choose a target device, and every tool will do exactly what you asked if you choose the wrong one. External drives, memory cards and second internal disks have all been erased this way by people who knew perfectly well what they were doing and clicked one line too high.
Three copies, two kinds of media, one elsewhere
The old rule still holds. Keep three copies of anything you care about, on at least two different kinds of storage, with at least one of them in a different physical place. The last part covers the failures that take out a room rather than a device: theft, fire, water, a power event.
Cloud storage counts as the offsite copy for most people. Synchronisation, though, is not a backup — a file deleted on your machine is deleted in the sync folder too. Use a service with version history and a deletion grace period, and know how to reach both.
The things people forget
- Browser bookmarks, saved passwords and extension settings.
- Email stored locally rather than on a server.
- Application licence keys and activation files.
- Signing keys and two-factor recovery codes.
- SSH keys and configuration files that begin with a dot.
- Virtual machine images and container volumes.
- Anything sitting on the desktop, which is a folder like any other.
Identifying the target with confidence
Before you write anything, establish which device is which. The reliable method is comparison: list the storage devices with the target unplugged, plug it in, list them again, and take the difference.
| System | Command or tool | What to match on |
|---|---|---|
| Windows | Disk Management, or wmic diskdrive list brief | Model name and exact size |
| Linux | lsblk -o NAME,SIZE,MODEL,TRAN | The usb transport and the size |
| macOS | diskutil list external | External devices only |
Unplug every removable device except the target before you begin. And read the confirmation dialog out loud — it names the device, and saying it aloud is a surprisingly effective interrupt.
Verifying that the backup is real
An unverified backup is a hypothesis. Test it, briefly, before you need it:
- Open several files from the backup, chosen from different folders — not just the first one in the list.
- Compare total size and file count against the source.
- Check that hidden files and configuration directories came across; many copy tools skip them by default.
- Restore one folder to a temporary location and confirm the contents are intact.
- Record the date and what was included, so you know later what the copy contains.
Full disk images versus file copies
A file-level copy is small, quick and easy to browse, and it suits documents and media. A disk image captures the whole device including the operating system and its configuration, and it suits a machine you would rather not rebuild from scratch. They answer different questions. For a system disk you are about to change, an image is the one that gets you back to where you started.
Before the write
Close applications that might hold files open. Confirm the device is the one you identified, once more. Make sure the machine will not lose power part way through — for a laptop, that means the charger. Then write, and let it finish; interrupting a write leaves a device in a state that is neither the old contents nor the new ones.