We want to share with you a note from our experience in converting a 320GB hard disk drive from MBR to GPT without data loss. We use gdisk, a famous tool by Roderick "Rodsbooks" Smith, and do several steps to finish it in a very quick time. Here's the results.

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1. Install gdisk:

Run your Terminal and do command line as the following. 

$ sudo apt-get install gdisk

2. Start terminal session with administrator access:

$ sudo -i

3. Check and note you hard disk drive list:

# lsblk

This should give you output like below. Our hard drive is /dev/sda in this example. 

master@master:~$ lsblk 
sda      8:0    0 298,1G  0 disk  
├─sda1   8:1    0  97,7G  0 part /media/master/penyimpanan
├─sda2   8:2    0 178,8G  0 part /media/master/data
├─sda3   8:3    0   286M  0 part  
├─sda4   8:4    0   1,9G  0 part  
└─sda5   8:5    0  19,4G  0 part /media/master/trisquel

master@master:~$ 

4. Run gdisk to your hard disk:

# gdisk /dev/sda

5. Write a GPT partition table:

# w

This should give you messages like below:

6. Reload the partition table. See picture above.

# partprobe /dev/sda

7. Update your bootloader. See also picture above. 

# update-grub

8. Reboot

Reboot and confirm your hard disk partition table with Disks. It should now reports "GUID Partition Table" (GPT) being used by your hard disk. See initial picture above.

References

Change partition from MBR to GPT, without data loss

Converting to or from GPT by the author of gdisk himself

Convert a disk from MBR to GPT+UEFI, in Linux. 

Convert MBR partition to GPT without data loss