Why does the Memory Stick slot become unusable when I click the [Unplug Hardware] icon on the task tray and execute [Stop USB Disk-Drive(S:)]?

If I click the [Unplug or Eject Hardware] icon on the task tray and select [Stop USB Disk-Drive (S)], it becomes impossible to use the Memory Stick.
After that, if I insert the Memory Stick in the state that [Memory Stick (S: drive)] is not recognized, [VAIO Action Setup] starts. If I have set [Explorer] on the [VAIO Action Setup] window as the software that starts when the Memory Stick is inserted, the following error message appears.

--Error message--
"S:\ refers to a location that is unavailable. It could be on a hard drive on this computer on a network or on a different computer on your home network. Check to make sure that the disk is properly inserted, or that you are connected to the Internet or home network and then try again.
If it still cannot be located, the information might have been moved to a different location."

In case that the Memory Stick is not usable, [S:Drive] is not accessible and [Stop USB Disk-Drive] has been executed, please refer to the procedure below:

--Procedure--
1. Double-click the [My Computer] icon on the desktop and double-click the [Control Panel] icon.
2. Double-click the [System] icon. If you cannot find the [System] icon, click [View all Control Panel options].
3. Click the [Device Manager] tab on the [System Properties] dialog box.
4. Click the [Refresh] button.