An architect’s iMac had started refusing to copy files, throwing macOS “Error code −36” and freezing, with years of AutoCAD drawings on board. That error is macOS reporting a read/write failure at the disk — usually bad sectors under a damaged HFS+ file system. Disk Utility couldn’t fix it, and every retry was stressing a dying drive. We imaged it around the bad areas, rebuilt the file system, and recovered the drawings.
An architect relied on an iMac for live project work — AutoCAD drawings, blueprints, floor plans and 3D models, built up over years. It began throwing “The Finder can’t complete the operation because some data can’t be read or written (Error code −36)” whenever certain files were copied or opened, alongside slow access and the occasional freeze. macOS Disk Utility’s First Aid ran but couldn’t repair the drive. Sensibly, rather than keep hammering First Aid and hoping, the machine was shut down and the drive sent to us — because on a disk that’s physically failing, each repair attempt is another lap the failing heads don’t need.
Error −36 is an old, low-level macOS input/output error (ioErr). Stripped of the jargon, it means the system asked the disk for some data and the disk failed to return it cleanly. On a drive this usually points to bad sectors — areas of the platter the heads can no longer read reliably — and/or a damaged file system whose records point at data that can’t be retrieved. It isn’t a cosmetic warning; it’s the drive telling you it’s struggling to read your files. Continuing to use the machine, or repeatedly running repair utilities, tends to grow the bad areas and can push a marginal drive over the edge.
Disk Utility’s First Aid checks and repairs the logical structure of the file system — it is not a tool for physical media faults, and it has no safe way to recover data from sectors the drive can no longer read. Worse, it works on the live, failing disk: it reads and writes to the very drive that’s already faltering, and if it meets structures it can’t reconcile it will discard or truncate them rather than preserve your data. So a repeated “First Aid failed” on a drive throwing −36 isn’t a mystery — it’s the wrong tool for the actual problem, and running it again risks making things worse. The right first move is always to stop and take a copy.
The drive was cloned to healthy storage through a hardware imager, write-blocked, with a profile built for failing media: read retries kept low so a bad sector is skipped quickly instead of being hammered, the healthy majority of the disk secured first, and the weak zones revisited afterwards in smaller passes — including reverse reads — to pull back as much as possible without stalling the heads. The bad areas were exactly where the architect’s recently-edited drawings lived, which is why those files had started failing first. Unreadable sectors were logged and mapped, and every later step was carried out on the image, leaving the failing original untouched.
macOS drives of this era use the HFS+ file system, whose index — the catalog file — is a B-tree holding the name, location and structure of every file and folder. Corruption there is a classic cause of files appearing blank, refusing to open, or returning −36. Working on the image, the catalog was rebuilt from the catalog B-tree and its backup structures, restoring the folder tree and the file records so the drawings could be located and named correctly. For files whose data ran through the bad sectors, signature carving recovered as much as could be read, and the affected DWG and DXF drawings were repaired at file level so AutoCAD would open them. Between file-system rebuild and carving, the great majority of the project archive came back intact.
Recovered drawings were opened and checked to confirm they loaded in AutoCAD, and the wider set of blueprints, models and documents was validated for completeness before sign-off. A small number of files sitting directly over the worst-damaged sectors could not be fully rebuilt, leaving a 98% recovery with all the client’s live project work restored. Everything was returned on a fresh external drive. We made the usual point too: −36 is often the first warning a drive gives before it fails outright, so the moment it appears is the moment to back up and stop using the disk — not the moment to keep working and hope it settles.
Hardware imager with write blocker and damaged-media profile (retry-limited, reverse-read passes) · HFS+ catalog B-tree reconstruction · signature carving for CAD formats · DWG/DXF file-level repair. Read-only imaging, all work in-house at our Belfast lab.
Stop running First Aid, shut the Mac down, and send us the drive for a free, no-obligation diagnostic. We’ll tell you what can be recovered and put a fixed price in writing before any work starts — and on most jobs it’s no fix, no fee. Post it in from anywhere in the UK, or drop it to us in Belfast.
It’s a low-level input/output error — macOS asked the drive for data and the drive couldn’t return it cleanly. On a hard drive that usually means bad sectors, often with file-system damage on top. It’s a sign the disk is physically struggling to read your files, so it’s best treated as an early warning to back up and stop using the drive.
First Aid only fixes the logical file system; it can’t recover data from sectors the drive can no longer read, and it runs on the failing disk itself. If it keeps failing on a drive throwing −36, running it again risks further loss. The safe route is to image the drive first and do any repair on the copy — which is how a proper recovery is done.
Yes. We rebuild the file system to locate the drawings, recover their data from the imaged disk, and where a file was damaged by bad sectors we repair the DWG or DXF at file level so it opens in AutoCAD. We check the drawings load before returning them, so you know what you’re getting back.