Why do my network drive mappings have an

After upgrading to Windows 2000 from NT 4.0, each of the four workstations that I upgraded have an “x” on their network mapped drives. For example, the Y: drive is mapped to \\Tech\support. I can double-click on the drive in My Computer and it opens fine, but sometimes if I am accessing that drive from an application, I get an error message saying that the drive is inaccessible. I have disconnected the drive and re-mapped it, but the x comes back. Please advise.