How to convert MBR disk to GPT?
Step 1: Launch PartitionGuru from your computer. Select hard drive you want to convert and click "Disk" to select "Convert to GUID Partition Table".
Step 2: Click "Yes" on the message box. Please note that only new operating system can use GPT disk, for example, 32-bit Windows XP does not support GPT disk. Also, only the EFI-based computer can boot from GPT disk. Thus your computer won't be able to boot normally if it doesn't support GPT and you need to reinstall Windows.
Step 3: Click "Save All" button to save changes to partition table and the conversion takes effect.
For more information: http://www.eassos.com/tutorial/hard-drive/mbr-vs-g pt.php