[Taken from http://www.astroarch.com/blog/?p=104#more-104]
Using ‘diskpart’ enter the following commands:
NOTE 1: The disk number you select varies. Windows uses a 0-based index for disks, so the system disk should be Disk 0, and additional disks will be 1, 2, 3, etc. If you go into the Disk Management (Server Manager screen -> Storage -> Disk Management) you should see all the disks, and it will give you each of their Disk numbers. Once you use all the commands above, they will go from offline to online in Disk Management.
DISKPART> SAN POLICY=OnlineAll
DISKPART> SELECT DISK 1
DISKPART> ATTRIBUTES DISK CLEAR READONLY
DISKPART> ONLINE DISK
DISKPART> CONVERT MBR
NOTE 2: The Convert MBR command may not work. That's fine. If it's not the primary/system/bootable disk, it won't have an MBR, so the command will most likely fail.