This post to show a Trick when you try to Start build in the TFS and you get this error when you try to start your build in TFS.
Make sure of the following :
1) the network path is created and shared.
2) not network connectivity problem.
And this trick is : this folder should be shared and give full previliges not to you even you are an administrator on the drop location,you have to give full access to the tfsSerivce account to that shared folder.