I think I had this problem before but I don't remember how it was fixed.
I have Norton from Comcast, Norton Security Suite v21.2.0.38. I just reset a computer for my wife from XP where it worked perfectly to VISTA and reinstalled everything from scratch. No more XP! Now I set up the daily backup just like I had it before to a mapped network drive on another computer. To get started, I try to run it manually and expect that the first backup will take a long time. However it only runs for a few minutes and then stops with an error, "there is a backup problem. Make sure the destination drive or device is connected" I can restart it and it does continue where it left off but again only runs for a short while before I get the same error. This is very frustrating.
BTW. I have exactly the same setup on my computer and it works perfectly. The same setup on my wife's computer worked perfectly while it was running XP. So I don't think I have any network problem in general. I am guessing it is some ethernet setting on VISTA but I don't know.
I keep restarting it but after 24 hours, it is only about 15% done.
Any suggestions. Thanks in advance.