This is a known issue with Windows Home Server. Please see
Microsoft KB Article 957825
"This update allows Windows Home Server to report the total drive space available rather than the space available on the primary drive. However, a side effect of the update is that mapped drives appear to report inaccurate information. That is, they report more space than the capacity of the drive."
Article ID: 143, Created On: 8/19/2011, Modified: 9/14/2011