Ethernet disk not available

[malc1]

I use a Lacie Network space 2 for media and backup for a windows 7 notebook and an iMac. Invariably the disk is not accessible from the Lacie Network assistant via the iMac even though the NAS is used for my iTunes library and usually functions for Sonos and iTunes.
The Networkspace does usually show up in Finder but not always
Conversely The NAS is always accessible in Network Assistant via the win 7 notebook.
I don't know where to start with this problem, sometimes Sonos can't access the NAS.
The drive is wired to the router. the iMac and Win 7 machine connect on wifi via devolo home plugs. There are tv's and tablet that are also on the network.
The tiltle of the post is what appears when starting Network Assistant on the iMac.