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Knowledge Center

Why are files beginning with "._" created when I copy or back up files to an external storage device?

Last updated:Nov 12, 2020

Why are files beginning with "._" created when I copy or back up files to an external storage device?

Symptoms

When you back up or copy a file from a volume to an external storage device (e.g., USB/eSATA drive or SD card), an additional file with the prefix "._" is created on the destination.

For example, when you copy and paste a file called "FILE.pdf", another file called "._FILE.pdf" is created alongside the copied file.

Diagnosis

This issue occurs when the following conditions are met:

  • The file was once accessed by a Mac computer via AFP or SMB.
  • The file system of the destination device is not ext4 or Btrfs (e.g., FAT32, exFAT, and NTFS).

Resolution

When accessing files on a Synology NAS, macOS may add auxiliary information associated with the files, known as "extended attributes" (EAs). File systems such as FAT32, exFAT, and NTFS do not support EAs. EAs are therefore split into separate files that bear the same names as the original files, but start with "._".

These "._" files on the destination device are no longer effective, so you can delete them manually.