Iis – No space on disk for .NET IIS App

I have an IIS server running multiple apps. When I checked the space available on the C:\ drive on that server it said 20GB free. But I still get the following stack trace:

 [IOException: There is not enough space on the disk.
 ]
System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath) +7717304
System.IO.Directory.InternalCreateDirectory(String fullPath, String path, DirectorySecurity dirSecurity) +7569557
System.IO.Directory.CreateDirectory(String path, DirectorySecurity directorySecurity) +150
batchprocessor.btnUpload_Click(Object sender, EventArgs e) +979
System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e) +111
System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument) +79
     System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +175
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565

Is there a separate quota somewhere?