On a Mac, error -36 is an input/output failure reading or writing the file, often from leftover AppleDouble metadata files on a non-Mac drive, or a bad disk sector. Jump to your situation below or work through the methods in order.
On macOS, error -36 is an input/output error, the system could not fully read or write the file. With an MBOX file it most often appears when the file has been on a non-Mac volume (a Windows-formatted drive, a network share or a USB stick) that scattered hidden AppleDouble files (the ._ companion files macOS uses to store metadata) and they have become inconsistent. It can also mean a genuine bad sector on the drive. The classic fix is the dot_clean command, which merges and clears those ._ metadata files so the Finder can read the real file cleanly. Beyond that, copying the MBOX to a healthy local APFS or HFS+ drive and running a disk check resolves the hardware and filesystem causes.
Error -36 is macOS reporting that a read or write did not complete. It is not specific to email, but MBOX files hit it often because they travel between systems during migration, and that travel is where the trouble starts.
The usual trigger is AppleDouble metadata. When a Mac writes to a non-Mac drive it creates hidden ._ files alongside each real file to hold Mac-specific attributes. If those get out of sync or partly copied, the Finder chokes on the pair and returns -36. Merging them with dot_clean, or moving the file to a native Mac drive, clears it.
“Error -36 looks like the disk is dying, but for MBOX files it is usually those little ._ ghost files that macOS leaves on non-Mac drives. The file came off a Windows stick or a NAS, the metadata twins got out of step, and the Finder refuses to read the pair. One command, dot_clean on the folder, merges and clears them and the copy goes through. If that does not do it, I get the file onto a real Mac drive and run a disk check.”
dot_clean /path/to/folder/containing/mbox
For an MBOX file, error -36 is usually the hidden ._ AppleDouble metadata from a non-Mac drive, so dot_clean is the first and most effective fix. Getting the file onto a native APFS or HFS+ drive avoids the problem entirely. Reach for a disk check only if dot_clean and a clean copy do not clear it, since then a real disk fault is likely.
If dot_clean and a disk check do not help and the copy still fails at the same point, the MBOX file has a damaged region on the source media. Use a tool like ditto or rsync that can continue past errors to salvage as much of the file as possible to a healthy drive, then import that copy. You can also submit your error to us for a tailored fix.
