Fritz NAS Problem


I tried to get access via Volumio to a USB HDD which is atached to my FritzBox.
IN Fritzbox GUI I generated an accountname and path on the HDD and of course a passwort.
The HDD could be seen from my Linux PC and other Windows PCs, after entering Name and Passwort.
But with Volumio I had no chance. I tried over hours, but couldn´t get any access at all. In most cases it tells me "mount: // is not a valid block device ". Where “WLAN-BEC” is the sharename which I defined in Fritz GUI. This name is also shown on one of my Linux PCs.
What I am doing wrong???
I am a Linux noob so please be patient. I couldn´t find anything helpful in the forum.