Data Recovery Case File · Mac & Apple Ecosystem · Both Halves or Nothing
Fusion Drive: One Volume Across Two Devices
His enquiry came with a diagnostic report already attached and quoted it accurately. A late-2013 iMac with a 3TB Fusion Drive that failed to boot, showing a dark screen with a circle and diagonal bar. Both halves had been removed and assessed, and the report read: "your drive array has a healthy 128GB SSD that can be readily imaged and worked with, but it has a damaged and failing headstack assembly" on the hard disk. That report is accurate and useful. What it needs alongside it is the thing that makes this case different from an ordinary failed drive — a Fusion Drive is not a fast disk and a backup disk. It is one volume physically distributed across two devices, and a healthy half is not half the data.
| Media | Late-2013 iMac 3TB Fusion Drive — 128GB solid-state module assessed as healthy and imageable; associated hard disk with damaged head assembly; both components removed |
| Reported situation | Machine failing to boot with a prohibitory symbol · both Fusion components removed and assessed elsewhere · solid-state half reported healthy · hard disk reported as having a failing head assembly · complete volume required |
| Fault class | Logical volume spanning two physical devices with one member mechanically failed — reconstruction requiring both members imaged and reassembled offline |
| Equipment used | Solid-state member imaged first, write-blocked · hard disk inspected under the laminar flow bench before power and serviced with matched donor parts · PC-3000 Express with Data Extractor prioritised imaging · logical volume reassembled offline from both images; losses mapped per file |
The decode: what a Fusion Drive is, and why both halves matter
How it actually works: a Fusion Drive combines a small solid-state module and a large hard disk into a single logical volume managed by the operating system. The system moves data between the two automatically — frequently-used material onto the fast module, everything else onto the disk — and presents the result as one drive. The user never sees two devices, and there is no fixed rule about which files live where. Crucially, the volume's own structures, including the records describing where everything is, are themselves distributed across both members.
Why that changes the arithmetic: people hearing "the SSD is healthy" reasonably assume a portion of their data is safe. It is not that simple. The 128GB module holds whatever the system had placed there, plus part of the volume's structure — and without the hard disk's contribution the volume cannot be assembled at all. Equally, the hard disk alone cannot be read as a normal drive, because the structures describing it are incomplete. Neither half is usable on its own. A recovery therefore requires both members imaged and the logical volume reassembled from those images.
What that means practically for his case: the healthy module is the easy part and should be imaged first, completely, because it is stable and there is no reason to leave it at risk while attention is elsewhere. The hard disk is the real work — a failing head assembly means inspection before power, matched donor parts, and prioritised imaging in one supervised session. Only once both images exist is the volume reassembled offline, where nothing can attempt a repair.
Why the report's wording is worth trusting: a shop that distinguishes between the two members, states one is readily imageable and identifies a head assembly problem on the other has done real diagnostic work and described it honestly. That is a useful handover, and it means the assessment starts from a known position rather than from scratch.
What must not happen: two things specific to this configuration. Nothing should attempt to split, repair or re-create the Fusion pair — macOS offers to do exactly that, and re-creating the pair writes fresh structures over the ones a reconstruction needs. And the machine should not be booted with either member attached, because the system may try to repair the volume it finds broken.
On the bench
The solid-state member was imaged first and write-blocked, since it was healthy, stable and there was no reason to leave the easy half exposed while the difficult one was worked on. The hard disk was not powered on arrival: it was opened under the laminar flow bench and its head assembly and platter surfaces inspected before anything was switched on, then serviced with matched donor parts. Imaging ran on the PC-3000 Express under Data Extractor, head-mapped and prioritised in one supervised session. Only then was the logical volume reassembled offline from both images, and any unreadable regions resolved to the files they affected.
The outcome
Both members imaged, the logical volume reassembled offline from the copies, and the losses reported per file. Free assessment, one fixed written figure including VAT, 50% of parts and labour upfront with the balance only on successful recovery. The decode, for anyone with a Fusion Drive: it is one volume physically spread across a solid-state module and a hard disk, with the system moving data between them automatically and the volume's own structures distributed across both — so a healthy SSD is not half your data, and neither member can be read alone; both must be imaged and the volume reassembled from those images; image the healthy half first because it is stable and easy; and never let anything split, repair or re-create the Fusion pair, because that writes over exactly what a reconstruction needs.
Fusion Drive with one half failing
Don't let anything split, repair or re-create the pair, and don't boot the machine with either half attached. A Fusion Drive isn't a fast disk plus a storage disk — it's a single volume physically distributed across both devices, with the system moving files between them automatically and the volume's own structures spread across the pair. That means a healthy solid-state half doesn't give you half your data; neither member is readable on its own, and both have to be imaged before the volume can be reassembled. Keep both components together and labelled, and don't discard the failing one as beyond hope — it's required. If you're offered a "repair" of the Fusion set by the operating system or a shop, refuse: re-creating the pair writes fresh structures over the ones a recovery reads. Image the healthy half first, since it's stable and costs nothing to secure.
Keep both halves — call Glasgow Data Recovery on 0141 404 0294; healthy member imaged first, failing disk serviced under inspection, volume reassembled offline from both images.
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.