JSI Tip 5274. You cannot Revert a disk to a Basic Disk if the disk appears as "Dynamic Unreadable" in Disk Management?

Microsoft Knowledge Base Article 320283 contains the following Description:

If the master boot record (MBR) of a disk is damaged or if it is improperly altered, the disk may appear as "Dynamic Unreadable" in the Disk Management snap-in. You can right-click the disk, and then click Revert to Basic Disk to change the disk...