This problem usually occurs in windows xp when a usb drive is usually plugged into a usb port but as some of the network permissions change overnight due to which the same drive that shown earlier as e: drive in windows explorer is not showing.
So now due to the network permission under E:\ and there is a conflict with the USb stick.
In short your removable drive or a normal usb drive is detected by the system but due to not being displayed in the windows explorer.
Lets see how you can figure out a simple solution to this problem.
1. Start >> Control Panel >> Administrative Tools >>Computer Management and now click on disk management.
2. Select the portable usb drive and right click on it and go to “change drive letter” then assign a new drive letter to it such as x,y,z to avoid any conflicts.
3. That’s it done! as soon as you assign a new drive letter i will show up in the windows explorer.
If you have any problem in following the above procedure leave a comment here i will be glad to answer you.