SYMPTOMS

On a computer that runs one of the versions of Windows that is listed at the beginning of this article, if you map a drive to a network share, the mapped drive may be disconnected after a regular interval of inactivity, and Windows 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.
 

CAUSE

This behaviour occurs because the systems can drop idle connections after a specified time-out period (by default, 15 minutes) to prevent wasting server resources on unused sessions. The connection can be re-established very quickly, if required.
 

RESOLUTION

To have us change the default time-out period for you on the server side, go to the "Fix it for me"  section. If you prefer to fix this problem yourself, go to the "Let me fix it myself" section.


NOTE: You cannot change the default time-out period for Microsoft Windows 95-based computers and Microsoft Windows 98-based computers that have shared resources.


To resolve this behaviour, change the default time-out   period on the shared network computer. To do this, use one of the following   methods.

Registry Editor

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.   

Use Registry Editor to increase the default   time-out period. To do this, follow these steps, and then quit Registry Editor:

NOTE: You cannot use this method to turn off the autodisconnect feature of the Server service. You can only use this method to change the default time-out period for the autodisconnect feature.     
  1. Click Start, click Run, type regedit (Windows 2000 or Windows Server 2003) or type regedt32 (Windows NT 4.0), and then click OK.
  2. Locate and then click the following key in the registry:
    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\lanmanserver\parameters
  3. In the right pane, click the autodisconnect value, and then on the Edit menu, click Modify. If the autodisconnect value does not exist, follow these steps:
    1. On the Edit menu, point to   New, and then click REG_DWORD.
    2. Type autodisconnect, and then press ENTER.
  4. On the  Edit menu, click  Modify.
  5. Click Hexadecimal.
  6. In the Value data box, type ffffffff, and then click OK.
NOTE: The client-side session is automatically disconnected when the idling time lasts more than the duration that is set in KeepConn. Therefore, the session is disconnected according to the shorter set duration value between AutoDisConnect and KeepConn. To change the time-out duration in the client-side during a UNC connection, specify the arbitrary time in KeepConn.
Locate and then click the following key in the registry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Service\lanmanworkstation\parameters
Value: KeepConn
Data type : REG_DWORD
Range : 1 to 65535 (sec)
Default value: 600 sec = 10 mins

Command Line

NOTE: If you use this method, you may turn off the autotuning feature   for the Server service.

To change the default time-out period for the   autodisconnect feature of the Server service, open a command prompt, type the   following line, and then press ENTER
net config server /autodisconnect:number
where number is the number of minutes   that you want the server to wait before it disconnects a mapped network drive.   The maximum value for this command is 65,535.

NOTE: If you set the autodisconnect value to 0 (zero), the autodisconnect feature is not turned off, and the   Server service disconnects mapped network drives after only a few seconds of   idle time.

To turn off the autodisconnect feature, open a command   prompt, type the following line, and then press ENTER:
net config server /autodisconnect:-1

http://support.microsoft.com/kb/297684