Here is a few options to archive this:
1. Use iStorage Server built-in encryption feature to encrypt data in server, which support Windows NTFS encryption and use defined encryption, however, by using this method, you can't encrypt data in the line (Ethernet link).
2. Use KernSafe DiskEncryptor or any third party disk encryption software to encrypt client data.
3. Build IP Sec between iStorage Server and its clients, we provide a whiter
paper so user can follow:
http://www.kernsafe.com/white-papers/istorage-server-and-ip-sec.aspx