ERROR:

can't cd into nextcloud data directory. Would like to be able to copy/move files in this directory to an external drive for backup.

Missing permissions

To cd into the directory as sudo

Bash shell commands like “cd” can't be “sudo'ed”, so do this:

$ sudo -i

now you can cd into the directory and do what you like.

to exit:

$ exit

To list all files:

https://central.owncloud.org/t/cant-create-or-write-into-the-data-directory-cant-write-into-config-directory-data-directory-mnt-data-is-invalid/1255

You have permission issues that the users which is running your webserver has no access to this folders. If you're on e.g. Ubuntu/Debian this can be checked by running the following command:

$ sudo -u www-data ls -la /path/to/folder

this command should return a list of files / folders instead of a “Permission denied” message.

in my case:

$ sudo -u www-data ls -la /var/www/data

See also:

https://community.bitnami.com/t/how-to-access-the-owncloud-user-data-folders/30531/4