When you connect to the Terastation via SFTP groups are ignored. You only see the folders for which your user has been explicitly authorised. If you belong to a group which has access to the folder you won't be able to access that folder.
Group permissions are recognised if you connect via Samba or AFP but not over SFTP.
Apologies if this is not the place to submit bug reports. Feel free to move this post to whatever part of the forum is appropriate.
