Data Recovery Case File · Formatted & Logical Faults · Setup Writes More Than You Think

The Reinstall Was on a Different Disk

His enquiry describes a sequence that should have been safe. Having reinstalled the operating system on a separate drive, he reconnected his 3TB data disk and found that "while it shows up in the file explorer, when trying to open it it prompts me" to format. Installing to one disk is not supposed to affect another, which is why this is confusing rather than obviously explicable. But installation routines write in more places than most people expect — and there is a specific, well-known behaviour that produces exactly this.

The decode: what setup may have written, and why the prompt means what it means

What the prompt actually says: not that the drive is broken. It means the system read the volume's structures and could not interpret them, so it offered the only remedy it has — writing fresh empty ones so the drive becomes usable. Usable means empty. He must not accept it, and having declined so far is the reason this remains recoverable.

The behaviour that explains this best: operating system installers do not necessarily confine themselves to the disk you select for the system. When more than one disk is connected during setup, the installer may place its boot and recovery partitions on a different disk from the one receiving the operating system — commonly the disk the firmware considers first, which is frequently the older data drive. Creating those partitions writes new partition-table entries and small partitions at the start of that disk, which is exactly where the existing filesystem's leading structures live. The data area is untouched; the description of it is overwritten.

Why that produces precisely his symptom: the drive still appears, because the partition table is valid — it has just been rewritten. The volume will not open, because the structures describing the filesystem are gone or displaced. And the system offers a format, because from its point of view there is a partition containing something it cannot read.

Why the outlook is good: what was overwritten is measured in megabytes at the front of a 3TB disk. Everything behind it is intact. And filesystems keep backup copies of their key structures at known positions away from the start of the volume, which a new partition at the front does not disturb — so the original filing structure can frequently be rebuilt from its own backup, returning files with folders and names.

The lesson for anyone about to reinstall: disconnect every drive except the installation target. Physically unplug them. It takes two minutes, and it removes the entire category of risk — including the variant where the wrong disk is selected during setup, which is the same accident with a worse outcome.

What not to do now: not accept the format, not run repair utilities, and not use the drive for anything.

On the bench

The format was declined and the drive removed from use, since accepting it writes fresh empty structures over the description of everything on the volume. The drive was imaged write-blocked before any reconstruction, and the leading region examined for installation-written structures — establishing whether setup had placed boot or recovery partitions on this disk rather than the installation target, which is the commonest explanation and overwrites exactly where the filesystem's leading structures sit. The previous filesystem was rebuilt from its surviving backup copies on the image, and files validated by opening.

The outcome

The format refused, the drive imaged before any reconstruction and the previous filesystem rebuilt from its surviving backup 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, for anyone whose data drive started demanding a format after a reinstall: installers do not always confine themselves to the disk you chose. With several drives connected, setup often places boot and recovery partitions on whichever disk the firmware considers first, writing new structures at the very start of it — which is where your filesystem's leading records live. Next time, physically disconnect every drive except the target.

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.