Cannot Copy Data to an External Drive on macOS

Description

A grey circle with a slash is shown when moving files to an external drive.

Common causes of this are:

The drive is formatted for Windows as NTFS.

A Windows formatted drive will mount on macOS, but can not be written to.
The drive may have become corrupted.

Apple provides the First Aid tool in Disk Utility. This tool can be used to try and repair a corrupted drive.

The drive may need to be reformatted if it can not be repaired.

Backup the data and format the drive to exFAT if the drive is NTFS.

exFAT formatted drives can be used on both macOS & Windows.

Details