Here is a short memo of how you can convert dynamic disk to basic from command line:

cmd> diskpart

DISKPART> list disk

DISKPART> select disk 0

DISKPART> detail disk

-- ------------------------------------------
-- note volume numbers on selected disk
-- and then delete all volumes from the disk
-- ------------------------------------------

DISKPART> select volume 3
DISKPART> delete volume

DISKPART> select volume 4
DISKPART> delete volume 

...
...

DISKPART> select volume 5  <-- last volume on disk 0
DISKPART> delete volume

-- ------------------------------------------
-- now, you can convert dynamic disk to basic
-- ------------------------------------------

DISKPART> select disk 0
DISKPART> convert basic