It’s usually the same story. You’re trying to move a massive folder of old photos or a 4K video file to an external drive, and just as the progress bar looks promising, everything grinds to a halt. A grey box pops up: "The Finder can’t complete the operation because some data in 'File Name' can’t be read or written. (Error code -36)."
It's frustrating. Infuriating, actually.
Most people assume their hard drive is dying when they see error code -36 mac pop up on their screen. While that’s a possibility, it’s rarely the actual culprit. This error is almost always a "Dot-Under-Bar" problem. It’s a conflict between how modern macOS handles files and how older file systems (like FAT32 or ExFAT) deal with hidden metadata. You're basically watching a communication breakdown in real-time.
What is Error Code -36 anyway?
Let’s get technical for a second, but not too boring. Mac files are more complex than they look. Every time you save a file, macOS creates a hidden companion file called a "resource fork." These are those annoying files that start with ._ (dot-underscore) which you only see when you plug your drive into a PC. They store things like custom icons, window positions, and metadata.
Error code -36 happens when the Finder tries to move these hidden files but gets confused by the destination drive’s formatting. Maybe the hidden file got corrupted during a previous transfer, or the external drive is just refusing to let the Mac write that specific type of data. It’s a "I don't know what to do with this extra stuff" error.
The "Dot_Clean" Solution You’ve Been Looking For
There is a tool built into your Mac that solves this in seconds. It’s called dot_clean. Most people never touch the Terminal because it looks like something out of a 90s hacker movie, but for this specific issue, it is the only reliable fix.
Open your Terminal (hit Command + Space and type "Terminal").
Once it’s open, you’ll want to type dot_clean followed by a single space. Don't hit enter yet. Now, find the folder or the entire external drive that’s giving you the -36 error. Drag that folder or drive icon directly into the Terminal window. The Mac will automatically type out the file path for you. It should look something like this: dot_clean /Volumes/ExternalDrive/MyPhotos.
Now hit Enter.
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What did that do? It basically merged those invisible ._ files with the actual data files. It cleans up the mess so the Finder doesn't trip over its own feet. Try copying your files again. Ninety percent of the time, this fixes the error code -36 mac issue immediately.
Why FAT32 and ExFAT are usually the villains
If you’re using a thumb drive or an older external HDD, it’s probably formatted as FAT32. This format is ancient. It was designed when "large" files were measured in megabytes, not gigabytes. FAT32 has a hard 4GB limit on individual file sizes. If you try to move a 5GB movie file, you might get a -36 error, though usually, macOS gives a "file too large" warning instead.
ExFAT is better, but it's still prone to file system errors if you don't eject the drive properly. Honestly, if you only ever use your drive on a Mac, you should be using APFS (Apple File System).
When the hardware actually is the problem
I hate to be the bearer of bad news, but sometimes the "data can't be read or written" part of the error message is literal. If dot_clean didn't work, we have to look at the physical layer.
- The Cable: I once spent three hours troubleshooting a -36 error only to realize the USB-C cable I was using was a cheap charging cable, not a high-speed data cable. Swap the cable. Just do it.
- The Hub: If you’re using a USB hub or a dongle, plug the drive directly into the Mac. Cheap hubs are notorious for dropping data packets during large transfers.
- Disk Utility: Go to your Applications > Utilities > Disk Utility. Select your external drive and click "First Aid." This will check for directory corruption. It’s like a quick check-up for your drive’s brain.
Permission issues are a sneaky culprit
Sometimes the Mac thinks you don't have the "right" to move the file. This happens a lot with files migrated from an older Mac or a different user account.
Right-click the folder you’re trying to move and select "Get Info." Look at the bottom where it says "Sharing & Permissions." If you see your username there, make sure it says "Read & Write." If you’re moving an entire external drive, look for the checkbox at the very bottom that says "Ignore ownership on this volume." Checking that box can bypass a massive amount of headache. It tells the Mac to stop being so paranoid about who originally created the file and just let the data flow.
Is your Mac too full?
This sounds stupid, but it’s real. If your internal Mac SSD has less than 10-15GB of free space, the system struggles to create temporary "swap" files during a copy operation. The Finder needs a little breathing room to manage the buffer. If your drive is red-lining on storage, you might get a error code -36 mac simply because the Mac has nowhere to store the metadata it’s processing mid-transfer.
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Delete your trash. Clear your Downloads folder. Give the machine some room to breathe.
Dealing with "Ghost" Files
Sometimes, the file causing the error isn't even a file you care about. It might be a .DS_Store file. These are tiny files macOS uses to remember how you liked to view your icons (list view vs. grid view). If one of these gets corrupted, the whole transfer fails.
If you’re feeling brave, you can use the Terminal to delete all .DS_Store files on a specific drive. The command is find /path/to/folder -name ".DS_Store" -depth -exec rm {} \;.
Don't worry, the Mac will just recreate new ones the next time you open the folder. It’s like clearing the cache on a web browser. It forces the system to start fresh.
What if it’s a NAS or Network Drive?
Moving files over Wi-Fi to a Synology or QNAP server? That’s a whole different ballgame. Error -36 on a network drive usually means the connection dropped for a millisecond. That’s all it takes.
If you’re on a network, try "Connecting to Server" using SMB instead of AFP. Apple is slowly killing off AFP (Apple Filing Protocol), and it’s notoriously buggy on newer versions of macOS Sequoia or Sonoma.
To do this, in Finder, hit Command + K. Type smb:// followed by your server’s IP address. Using the SMB protocol is much more stable and often sidesteps the metadata conflicts that trigger the -36 error.
Nuance: The SD Card Problem
Photographers see this error more than anyone else. You take your SD card out of your Sony or Canon camera, stick it in your Mac, and try to import. Error -36.
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This usually happens because the camera’s internal database file is slightly different from what the Mac expects. Never delete files from your SD card using your Mac's Finder. It leaves "trash" files on the card that the camera doesn't understand. Always import your photos, then use the "Format" function inside your camera to wipe the card. It keeps the file structure clean and prevents the -36 error from ever appearing.
Summary of Actionable Steps
If you are staring at that grey box right now, follow this exact order:
- Run the dot_clean command: This is the highest-percentage fix. Open Terminal, type
dot_clean, drag your folder in, and hit Enter. - Check your hardware: Swap the USB cable and bypass any hubs. Direct connection is king.
- Permissions check: In "Get Info," make sure you have "Read & Write" access and try checking "Ignore ownership on this volume."
- First Aid: Use Disk Utility to scan for errors on both the source and destination drives.
- Small Batches: If you're moving 1,000 files, try moving 100 at a time. This helps you identify if it's one specific corrupted file causing the hang-up or a global system issue.
You don't need to buy "Mac cleaner" software to fix this. Don't fall for the ads. This is a file system quirk that can be handled with built-in tools and a little bit of patience. Most of the time, your data is perfectly safe; it's just the "invisible" parts of the file that are acting up. Fix the metadata, and the files will follow.