You may be prompted to format a volume that is already formatted when you manually assign a drive letter

Source: Microsoft Support

RAPID PUBLISHING ARTICLES PROVIDE INFORMATION DIRECTLY FROM WITHIN THE MICROSOFT SUPPORT ORGANIZATION. THE INFORMATION CONTAINED HEREIN IS CREATED IN RESPONSE TO EMERGING OR UNIQUE TOPICS, OR IS INTENDED SUPPLEMENT OTHER KNOWLEDGE BASE INFORMATION.

You are running a Windows Server 2008-based computer and you�manually assign a drive letter to a volume�using the following steps:

1. 
   You�run the Disk Management administrative tool, or run the Computer Management administrative tool and select the Disk Management action.
2. 
   You right-click a Volume and select Change Drive Letter and Paths.

You will see a message prompting you to confirm whether you want to�format the volume.� The popup message reads:

You may see this prompt even if the volume is already formatted and contains data.

If�you accept this prompt (which is the default selection in the dialog),�the volume will be reformatted, and�you will no longer be able to access�the data that the volume originally contained.

This problem may occur as the result of an internal Windows error.

Under certain�conditions, the Shell Hardware Detection Service�may encounter�errors in retrieving volume information when a volume mount event�is detected.� Such errors�may�result in�the state of the volume being reported incorrectly.� As a result, the volume may be incorrectly�reported as unformatted when in fact it is already formatted and contains data.

When you�assign a drive letter to a volume�that is�already formatted, and you are prompted by the system to format the volume,�do not proceed with the format request.� Click Cancel to dismiss the popup message.� The drive letter will be assigned, but the volume will not be reformatted, and the existing data will be retained.

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