Trying to Back Up to SSK External Drive

I am not a computer guru. So bear with me. I connected an SSK external drive to my network by connecting to the wireless router. I can see the External Drive in Windows, access the files, etc. I mapped the External Drive to Disk "Z". When I try to do an Acronis backup however, it says "Connection Failed" every time, even though I added the correct credentials. What should I do. Not too computer savvy so the explanation would be appreciated in plain terms. Thanks. Michael