GPT and MBR contain partition tables, which are used to define where partitions -- individual divisions of space that can function like separate storage devices -- exist on a disk drive. Most operating systems use MBR, although support is growing for GPT. If you have a computer that has multiple operating systems installed, but one operating system doesn't support GPT partitioning, you can convert GPT to MBR using gdisk, a partition tool used to manipulate the partition tables in Linux. Gdisk can convert a maximum of four partitions, as is the partition maximum for MBR.
Click "Applications." Point to "Accessories," then click "Terminal." Input the following into Terminal: sudo fdisk -l
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Press "Enter" to see the disk devices connected to the computer. The device with the asterisk next to it is the boot device. Write down the filename (for example, /dev/sda) assigned to the disk.
Input the following into Terminal: sudo apt-get install gdisk. Press "Enter" to install the partitioning tool to Ubuntu.
Input "sudo gdisk" into Terminal, then press "Enter" to run gdisk as a superuser. Input the name assigned to the boot device, then press "Enter" again.
Press "r," then hit "Enter," to switch to the recovery and transformation options. Press "g," then press "Enter," to select the option to convert GPT to MBR on Linux.
Press "0," then press "Enter," to convert GPT to MBR on the primary partitions. Press "y," then "Enter," to finalize and exit.