Model : Archer C7

Hardware Version : Not Clear

Firmware Version : latest

ISP :

Hi there. I have a strange issue with the USB sharing service. There is an external HDD on usb which is working fine on my home network. I can download/upload and streaming media files to my devices. Great. BUT, when i am outside, i can access those files via FTP with some ftp clients, downloading to my work PC for example, but the strange thing is if i try to upload some files to the attached HDD on the router it gives me an error (access denied)!!
I tried all kind of security settings on Archer´s configuration but nothing helps.

What am i missing here? Could be an security issue on the HDD? I tried also with a normal USB stick on USB nr2, and getting same error ( downloading ok, but no uploading files to)
I really need some advice, as i need to upload some files from my work remotely to the HDD attached to the router...

Any help??

Thanks