External Storage: Not allowed, because you have no permission to create subfolders

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I allowing users to use an external storage, but i cannot create subfolders on this storage.

Or for longer, use three backticks above and below the code snippet:

Heres what the permissions says on my /nextcloud/data folder:
drwxrwx---  6 www-data www-data  4096 Mar  2 20:03 data

Here is the permission on the storage itself:
drwxrwxrwx 3 www-data                www-data                4096 Mar  2 21:13 hdd1

Heres what i did in Nextcloud:

This is what happens, when i try to upload some folders to the external harddrive in Nextcloud via the Windows Client:

Nextcloud version (eg, 12.0.2): Nextcloud 13
Operating system and version (eg, Ubuntu 17.04): Linux raspberrypi 4.14.22-v7+ #1096 SMP Mon Feb 26 19:14:22 GMT 2018 armv7l GNU/Linux

Apache or nginx version (eg, Apache 2.4.25):
Server version: Apache/2.4.25 (Raspbian)
Server built: 2017-09-19T18:58:57

PHP version (eg, 7.1): 7.0

It’s probably because the webserver-user (local external storage) or the user of the external storage has no permissions (in this external storage) to create subfolders. So you need to grant these permissions on the external storage.

Without more information, we can’t do anything but to repeat the error message.