When you restart a computer that is running Windows MultiPoint Server (WMS) 2011, an external USB storage device may be assigned a different drive letter.
To resolve this problem, install the update rollup that is described in the following Microsoft Knowledge Base article:
2555129 Update Rollup 1 for Windows MultiPoint Server 2011
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates