You drag a folder from your Mac to an external SSD or USB drive connected through a USB-C hub.

The transfer begins normally—then Finder suddenly stops and reports:

“The Finder can’t complete the operation because some data can’t be read or written. (Error code -36)”

Apple’s Core Services documentation identifies ioErr as error -36, so this is fundamentally a file input/output error. However, the error number alone does not tell you whether the problem is the file, disk, metadata, cable, hub, or file system.

The fastest solution is therefore not to replace the hub immediately.

First determine which of these four patterns matches your problem.

Which Error -36 Pattern Do You Have?

This distinction matters because Error -36 is not automatically a USB-C hub failure.

Pattern A: One File or Folder Always Triggers Error -36

Suppose 50 files copy successfully, but the same folder fails every time.

That strongly suggests you should investigate the folder or its metadata before troubleshooting the entire USB setup.

Test the Problem File Separately

Create a temporary folder on your Mac desktop and copy only the failing item into it.

Then test:

Mac Internal Storage
        ↓
External Drive

If one particular file repeatedly produces Error -36 while other files copy normally, inspect:

• Whether the file opens normally
• Whether the filename contains unusual characters
• Whether the file is locked
• Whether another app currently has the file open
• Whether you have permission to read the source
• Whether you have permission to write to the destination

Apple’s current Mac guide recommends checking whether the file is open, whether the destination has enough free space, whether the external drive uses a writable format, and whether permissions allow the move or copy.

When ._ Metadata Files May Be Involved

macOS can store additional file metadata and resource information alongside ordinary files.

When Mac files are moved onto some non-native file systems, hidden files beginning with:

._

may be created.

Apple has documented that these ._ files can cause problems in certain workflows and that the built-in dot_clean utility can remove or consolidate problematic AppleDouble metadata.

If Error -36 consistently points to one folder, dot_clean is worth testing after making sure important data is backed up.

How to Run dot_clean

Open:

Applications → Utilities → Terminal

Type:

dot_clean 

Do not press Return yet.

Drag the problem folder from Finder into Terminal so macOS automatically inserts its path.

You should see something similar to:

dot_clean "/Volumes/My Drive/Project Folder"

Press Return.

Then try the Finder copy again.

Apple’s developer documentation specifically identifies dot_clean as a utility for dealing with problematic ._ files created when Mac files are stored on non-HFS+ file systems. See Apple’s official explanation of dot_clean and AppleDouble files.

Do not run random Terminal commands copied from forums on the entire system drive. Apply dot_clean only to the folder you are actually troubleshooting.

Pattern B: Many Files Fail on the Same External Drive

If different folders all produce Error -36 when copied to the same SSD, hard drive, or USB flash drive, the destination deserves more attention.

Start with the file system.

Check the Drive Format

Open:

Applications → Utilities → Disk Utility

Select the external volume and check Format.

Common formats include:

Apple’s Disk Utility documentation lists APFS, Mac OS Extended, FAT, and exFAT as supported formatting options. Apple also notes that NTFS drives are normally read-only on Mac, so Finder cannot simply write files to an NTFS destination without additional software.

If you can read files from the drive but cannot reliably write new ones, the problem may therefore be the format or permissions rather than the USB-C hub.

If the drive has become broadly unwritable rather than producing Error -36 for one file, see External SSD Becomes Read-Only Through a USB-C Hub for the separate NTFS, permissions, and write-protection workflow.

Run Disk Utility First Aid

If the drive format is appropriate but Error -36 appears across multiple folders, check the disk structure.

In Disk Utility:

1. Choose View → Show All Devices.
2. Select the external volume.
3. Click First Aid.
4. Run the check.
5. If required, repeat for the related container and physical device.

Apple recommends using First Aid to check and repair formatting and directory-structure errors on storage devices. If Disk Utility reports that a disk is failing, Apple recommends backing up the data and replacing the disk rather than continuing to rely on it.

You can link readers directly to Apple’s Repair a storage device in Disk Utility on Mac.

Do not erase or reformat the drive as your first troubleshooting step. Reformatting destroys existing data.

Pattern C: Direct Copy Works, but the USB-C Hub Fails

This is the pattern that most strongly implicates the connection path.

Test exactly the same file twice.

First:

Mac → External Drive

Then:

Mac → USB-C Hub → External Drive

Use:

• The same file
• The same external drive
• The same storage cable
• The same Mac
• The same destination folder

If the direct copy completes repeatedly but the hub path produces Error -36, investigate the hub connection.

Check the Port First

Make sure the storage device is connected to an actual data port.

Look for labels such as:

• USB 3.0
• USB 3.1
• USB 3.2
• 5Gbps
• 10Gbps
• USB-C Data

Do not connect an external SSD to a USB-C port marked only:

• PD
• PD IN
• Charging
• Power Delivery

A PD input may look identical to a USB-C data port while serving a completely different purpose.

Remove Everything Except the Drive

Temporarily simplify the connection:

Mac
 ↓
USB-C Hub
 ↓
External SSD

Disconnect:

• Second SSDs
• Card readers
• Webcams
• Capture cards
• Phones
• USB audio interfaces
• Other high-load USB devices

Then repeat the same file copy.

If Error -36 disappears, reconnect accessories one at a time.

The hub may not be “bad”; you may instead be exposing an unstable shared USB or power configuration.

If the external SSD disappears from Finder entirely rather than simply stopping one file operation, use External SSD Not Showing Up Through a USB-C Hub for the port, cable, power, and mounting checks.

Pattern D: Large Transfers Fail After Several Minutes

This is different from a folder that fails instantly.

Imagine:

2 GB copied successfully
↓
10 GB copied successfully
↓
Transfer continues
↓
Drive disconnects
↓
Finder Error -36

That pattern points more strongly toward sustained I/O stability.

Check:

• Storage cable
• Hub-to-Mac connector
• SSD enclosure
• Drive temperature
• Available power
• Other active USB transfers
• Whether the drive disconnects from Finder
• Whether macOS reports an unsafe ejection

A connection can appear completely normal during a short test yet fail under a long continuous write.

Look for the Important Clue

After Error -36 appears, immediately check Finder.

Drive is still mounted?

Focus on:

• Specific files
• File system
• Disk errors
• Permissions

Drive disappeared?

Focus on:

• USB connection
• Cable
• Hub
• Power
• SSD enclosure

macOS says “Disk Not Ejected Properly”?

That is a separate connection-stability symptom. Follow Mac Says “Disk Not Ejected Properly” When Using a USB-C Hub for sleep, power, cable, and disconnect testing.

Try a Different Storage Cable

Do not overlook the short cable between the external SSD and hub.

A damaged or marginal cable may:

• Mount the drive normally
• Allow small files to copy
• Fail during sustained writes
• Drop the USB link briefly
• Reconnect immediately afterward

That brief interruption may be enough for Finder to stop the operation.

For testing, remove:

• USB extension cables
• Magnetic USB adapters
• USB couplers
• Additional hubs
• Unnecessary converters

Use the simplest connection possible.

Check Free Space and Permissions

Before moving into hardware troubleshooting, confirm the basics.

Select the destination drive in Finder and choose:

File → Get Info

Check:

• Available capacity
• Sharing & Permissions
• Whether the volume is locked
• Whether your account has write access

Apple specifically includes free space and permissions among the checks to perform when Finder cannot move or copy an item.

If the problem occurs only with one user account or one protected folder, permissions are more likely than the hub.

What Not to Do First

Avoid these reactions immediately after seeing Error -36:

Do not immediately format the SSD.
You may destroy recoverable data.

Do not immediately replace the hub.
A specific corrupted file can create the same symptom.

Do not repeatedly unplug the drive while Finder is writing.
That can create additional file-system problems.

Do not run destructive Terminal commands from random forum posts.

Do not assume “the drive mounts” means the entire connection is stable.
A sustained copy test provides much more information.

The 10-Minute Error -36 Isolation Test

If you want the shortest troubleshooting route, use this order:

Minute 1–2

Copy another file to the same drive.

Only one file fails → investigate the file or metadata.

Minute 3–4

Copy the problem file while the drive is connected directly.

Direct works → investigate the hub path.

Minute 5

Check the external disk format and free space.

Minute 6

Remove all other devices from the hub.

Minute 7

Try another storage cable.

Minute 8

Run dot_clean on the affected folder if hidden metadata is suspected.

Minute 9–10

Run Disk Utility First Aid.

This order prevents you from making major changes before you know whether the failure follows the file, the drive, or the connection.

Final Verdict

Finder Error Code -36 means macOS encountered an I/O problem while trying to read or write data. Apple identifies ioErr as error -36, but the code itself does not identify one universal root cause.

Use the behavior of the error to narrow it down:

One file fails repeatedly:
Check the file and Mac metadata.

Every file fails on the same drive:
Check format, permissions, free space, and disk health.

Direct connection works but the hub fails:
Check the data port, storage cable, power, and hub connection.

Large transfers fail after several minutes:
Look for USB disconnects, unsafe ejection, heat, or sustained I/O instability.

The most useful diagnostic question is:

Does Error -36 follow the file, the drive, or the USB-C connection?

Once you know that, the troubleshooting path becomes much shorter.

FAQ

What does Finder Error Code -36 mean on Mac?

Apple documents ioErr as error code -36. In practical Finder use, it indicates a read/write problem during a file operation.

Can a USB-C hub cause Error -36?

A hub can be involved if the same file copies normally with the drive connected directly but fails through the hub. Test direct and hub connections before deciding that the hub is the cause.

Can dot_clean fix Error -36?

It may help when the problem involves ._ AppleDouble metadata files. Apple documents dot_clean as a utility that can remove problematic ._ files in relevant Mac file workflows. It will not fix a failing SSD, damaged cable, or unstable USB connection.

Does Error -36 mean my SSD is damaged?

Not necessarily. First determine whether the failure occurs with one file, one disk, or only one connection path. If Disk Utility reports that the device is failing, however, Apple recommends backing up the data and replacing the disk.

Why does the file copy directly but not through my USB-C hub?

That result makes the intermediate connection more suspicious. Check the hub data port, SSD cable, peripheral load, and whether the drive briefly disconnects during the transfer.

Should I reformat the drive?

Only after backing up important data and confirming that file-system format is actually the issue. Reformatting should not be the first response to Error -36.