Files on a Windows 2003 server share disappear

I have a Windows 2003 server with a USB ext. hard drive connected. We have a sharedx folder for users to save files to. The problem is after awhile the users can connect to the share but they can’t see any files. I can see the files when i am logged into the server and the share is up and running. If i reboot the server the users are able to see the files. Any ideals as to why the files disappear?