Location is not available Z:\ refers to a location that is unavailable

I have a network drive mapped to Z:. I can see all the files and folders on it in windows explorer. I can create new folders and files on it and read from it but when I try to copy files to it from my C: drive it says:

"Location is not available - Z:\ refers to a location that is unavailable. It could be on a hard drive on this computer or on a network. Check to make sure that the disk is properly inserted or that you are connected to the Internet or your network, and then try again. If it still cannot be located the information might have been moved to a different location."

Why? I can see it right here in windows explorer. It's so annoying and of course it makes me type in my administrator password before it pukes out the error. What can I do to fix this and yes I've rebooted my computer. It has worked in the past all of a sudden it starts doing this? I've disconnected and remapped network drive and rebooted in between doing those too but no change. Any ideas?

Have you tried to disable the firewall on your A/V software?