External storage, permissions problem

I have added an external disk to Nextcloud, but I am not allowed to move file & folder to another folder in the same external disk. My operating system is Ubuntu. And I have executed permission… on it.
What is the problem and what should I do?

Sometimes External Storage does not good. Have you set all directorys and files to your Nextcloud user and Nextcloud group e.g. www-data:www-data? What kind of external storage do you use? Post also logs from /path/to/your/nextcloud/data/nextcloud.log.