Mac error code -36: what it means and how to fix it
Error -36 is ioErr, an input/output error: the Finder started reading or writing a file and the operation failed partway. Its most famous habitat is copying files to a FAT32-formatted USB drive or memory card, where macOS's hidden companion files (the ones whose names start with ._) trip the copy. But -36 has a serious side too: on a Mac-formatted drive with ordinary files, repeated -36 errors can be an early report of a failing disk, which is why this page ends with a health check rather than only tricks.
What causes it
- Hidden ._ companion files that macOS creates for foreign filesystems, which can corrupt or fail on FAT volumes.
- A destination drive with errors, bad sectors, or a filesystem in need of repair.
- A cable, hub, or port dropping the connection mid-copy.
- A source file that is itself damaged or sits on a failing drive.
Fix it, in order
- Note which file stops the copy. Copy in smaller batches until one file reliably triggers the error. That file, or its ._ companion, is where the problem lives.
- Run the dot_clean command for FAT drives. In Terminal, the built-in dot_clean tool merges the hidden companion files with their real files for a folder: type dot_clean, a space, then drag the folder (or the mounted drive) into the Terminal window and press Return. Then copy again. This is the classic, specific cure for -36 on FAT media.
- Run Disk Utility First Aid on both the source and destination volumes. Filesystem errors are a real cause here, and First Aid is designed to find and repair them.
- Change the physical path. Different cable, direct port instead of a hub. An I/O error that vanishes with a cable swap was never about the files.
- Copy the file somewhere else as a test. If one file fails to copy to every destination, the source file or its disk area is damaged; recover what you can of it first.
- Treat recurring -36 on a Mac-formatted drive as a warning. Back up the drive promptly, then investigate its health. I/O errors are how failing drives introduce themselves.
Priorities matter with -36. If the drive involved holds the only copy of anything important, back it up before experimenting with repairs. First Aid is safe in normal use, but a truly failing drive can get worse under any workload, repairs included.
Common questions
What is dot_clean and is it safe? dot_clean is a command built into macOS that reconciles the hidden ._ metadata files macOS stores on foreign filesystems with their real files, removing the broken companions that stop copies. It is an Apple-supplied tool designed for exactly this error, and it does not touch the contents of your actual files.
How do I know if -36 means my drive is failing? Context decides. A -36 only when copying to one FAT USB stick, cured by dot_clean, is the companion-file quirk. A -36 appearing with different files on a Mac-formatted drive, alongside slowness, clicking, or files vanishing, is the failing-drive pattern, and backup comes before all other steps.
Related errors