WMI Queries of Win32_DiskDrive for the properties medialoaded and signature are not properly reported for non admin users of Windows XP SP2. This behavior is not seen in Server 2003, and I have not tested it with Vista. I have an open SRX with Microsoft for this issue, but expect to have it closed with only a KB article.
The demo script showing this problem is NewWMITest.vbs. I really don’t need signature data for a non-admin user, and used size >0 in place of medialoaded = TRUE for the WMI query.