On a computer that runs Windows 2000 or 2003, if you map a drive to a network share, the mapped drive may be disconnected after a regular interval of inactivity, and Windows File Explorer may display a red "X" on the icon of the mapped drive. However, if you try to access or browse the mapped drive, it reconnects quickly. This behavior occurs if the network share computer runs Microsoft Windows NT 4.0, Windows 2000, or Windows Server 2003.

The following Microsoft KB article explains how to resolve this issue: