No, you can's use Partition Assistant to change the disk from MBR to GPT without losing data. A conversion will automatically wipe the whole space of the drive. Firstly, backup the data and the convert the drive to the GPT standard. To do this, connect the drive, then go to Start > Run > diskpart. Input List disk then select disk X where X is the number of the 3TB disk. after the disk has been selected, input convert GPT and wait for the operation to finish and that's it. Copy the data back and the disk will remain with the GPT system.