Data Recovery Case File · Portable Drives · The Worst Moment to Attach Anything
An Installer's Job Is to Prepare Storage, and It Cannot Tell Which Disk You Meant
His enquiry describes a drive that was in the wrong place at the wrong moment. A backup of a Mac held on an external drive, which "was wrongly connected to a new computer that was being set up" with a fresh system installation. A setup process exists to find storage and prepare it — and an unfamiliar filesystem it cannot read looks, from where it stands, like a disk that is available.
| Media | External hard drive holding a backup written by a different platform — attached to a machine during operating system installation; filesystem not recognised by the installer |
| Reported situation | External drive holding a backup created on one platform · drive connected to a different machine during operating system setup · installation proceeding with the drive attached · backup content required |
| Fault class | Installer-initiated preparation of an attached volume — extent determined by whether the drive was selected, initialised or merely present |
| Equipment used | Extent of installer activity established before any conclusion · written regions measured against capacity · imaged write-blocked before any reconstruction · original filesystem structures located from surviving copies · files validated by opening |
The decode: what a setup process does with the disks it finds
What an installation is trying to achieve: a working system on usable storage. So it enumerates everything attached, works out what it can use, and offers to prepare it — which is exactly the behaviour that makes installation possible on a new machine with a blank disk.
Why an unfamiliar filesystem is the dangerous case: a drive written by another platform is not readable by the installer. From its position that space is unformatted or unrecognised, which is indistinguishable from empty — and empty space is what it is looking for.
What the possible outcomes are, and they differ enormously. If the drive was merely present and not selected, nothing may have happened to it at all. If it was initialised as part of preparing the machine, its partition structure was replaced. If it was chosen as the installation target, part of it has a system written across it.
Why establishing which happened is the first job: those three are a nuisance, a bounded structural loss, and a substantial overwrite respectively. The written extent is measurable against capacity, which distinguishes them without guesswork.
Why even the worst case is not total: an operating system installation writes a bounded quantity. On a drive of any modern size it occupies a fraction, written from the start of available space, leaving the remainder untouched — so content beyond it is physically present and undescribed.
Why the cross-platform detail helps rather than hurts: the original filesystem is a known structure with recognisable markers, and it keeps backup copies of its key structures away from the primary set. Where those survive beyond whatever the installer wrote, the original layout is read directly rather than deduced.
The general warning, which is worth more than this case: disconnect everything before installing an operating system. Not because setup processes are careless, but because their entire purpose is to find storage and make it usable — and they cannot know which disk holds twenty years of your photographs.
What must not happen now: nothing further written to the drive, and no acceptance of any offer to format or initialise it.
On the bench
The extent of installer activity was established before any conclusion — a setup process enumerating attached storage and offering to prepare it, with an unfamiliar filesystem presenting as unrecognised space and therefore as a candidate, producing materially different outcomes according to whether the drive was left alone, initialised, or selected as the target. Written regions were measured against capacity, an installation occupying a bounded fraction written from the start of available space.
The outcome
The extent of installer activity established, written regions measured against capacity, and original structures recovered from their surviving copies. Free assessment, one fixed written figure including VAT; where a drive has to be opened, 50% of parts and labour is payable upfront with the balance only on success — otherwise no recovery, no fee. The decode: an installer's purpose is to find storage and prepare it, and a filesystem it cannot read looks like space that is free. How much was written depends on whether your drive was selected, initialised, or simply present.
Before installing an operating system on any machine
Disconnect every drive except the one you're installing to — that single habit prevents this entirely. A setup process exists to find storage and make it usable, so it enumerates everything attached and offers to prepare it, and a drive written by another platform reads to it as unrecognised space, which is indistinguishable from empty. If it has already happened, write nothing further and refuse any offer to format or initialise. The outcome depends on whether your drive was merely present, initialised as part of setup, or chosen as the target — and even the worst of those writes a bounded fraction of a modern drive.
Write nothing further — call Oxford Data Recovery on 01865 593000; extent of installer activity established first, written regions measured against capacity, original structures recovered from surviving copies.
Request a quote online →
Our case files are drawn from genuine enquiries received by our laboratory over the past ten years, anonymised to protect client confidentiality. Each one describes the diagnostic and recovery procedure our engineers apply to that fault, using the equipment listed.