It seems that the external hard drive ejects automatically while you are transferring files, then Mac can't detect the drive and pops up the Device Disappeared error message. Solutions such as removing the USB hub, updating macOS, resetting Mac, etc., could work to get rid of this problem on your Mac.
If only those six files could not be successfully transferred using your Mac, then those files are probably damaged. You can try the data recovery software - iBoysoft Data Recovery for Mac to freely scan the drive and preview recoverable files.
After recovering data or backing up the drive, you can proceed to fix the external hard drive keeps disconnecting on Mac issue. Various reasons could lead to the external hard drive ejecting abnormally and triggering the error message "This operation can't be completed because the device disappeared" while attempting to move files. Then, you can try the following fixes:
- Plug the drive back in again
- Restart your Mac
- Disable the setting "Put hard drives to sleep when possible."
- Change between a direct connection and a hub
- Set the maximum disk sleep time as the default
- Turn off or remove the third-party antivirus software
- Boot into Safe Mode
- Use Disk Utility to First Aid the disk.
- Upgrade macOS
- Reformat the disk