Tak sorry, fakt som to nejak prehliadol...
Keď pravým klikneš na ten disk nemáš v ponuke: Delete Partition alebo Delete Volume. ?

Edit: Ešte môžeš skúsiť príkazový riadok:

To change a master boot record disk into a GUID partition table disk using a command line

1. Back up or move the data on the basic master boot record (MBR) disk you want to convert into a GUID partition table (GPT) disk.

2. Open a command prompt and type diskpart. If the disk does not contain any partitions or volumes, skip to step 6.

3. At the DISKPART prompt, type list volume. Make note of the volume number you want to delete.

4. At the DISKPART prompt, type select volume <volumenumber>.

5. At the DISKPART prompt, type delete volume.

6. At the DISKPART prompt, type list disk. Make note of the disk number of the disk that you want to convert to a GPT disk.

7. At the DISKPART prompt, type select disk <disknumber>.

8. At the DISKPART prompt, type convert gpt.