According to the help article on the Microsoft website (
https://technet.microsoft.com/en-us/library/cc755238.aspx) you need to:
To change a dynamic disk back to a basic disk using the Windows interface
Back up all volumes on the disk you want to convert from dynamic to basic.
In Disk Management, right-click each volume on the dynamic disk you want to convert to a basic disk, and then click Delete Volume for each volume on the disk.
When all volumes on the disk have been deleted, right-click the disk, and then click Convert to Basic Disk.