MBR stands for Master Boot Record is possibly the most important piece of data stored on your hard drives or USB flash drives. Basically, it is a dedicated small section of the drive that holds vital information of the disc including its boot information and the layout of all the partitions stored on it. MBR is created when you create the first partition on the hard drive. It contains the partition table for the disc as well as small amount of executable code for boot start and is highly important data structure on the disk. MBR contains information right at the beginning of the disc. MBR scheme is used to separate hard drives into distinct data area.
It is not possible to partition to external hard drive which is MBR formatted generally on Mac OS X EI Captain and Mac OS Sierra. In case of MS-DOS formatted external hard drive, Disk Utility on Mac OS X EI Captain and Mac OS Sierra does not allow you to add, resize and delete partitions on external hard disk drive. The main reason behind Disk Utility not allowing you to partition external hard disk drive formatted with files system other than HFS is that these drives contain “Master Boot Record”, a partition scheme commonly used by Windows PC. In order to do so, you need to re-format the external storage media to GUID partition table then only Disk Utility will allow you to add, resize or delete partitions.
When you try to partition the MBR external hard drive through Disk Utility, you cannot do so because all the partition buttons and task buttons are disabled. For manual partitions through Disk Utility, you require to reformat the entire hard drive into GUID partition i.e, OS X Journaled. Once the GUID partition mapping is done, Disk Utility will enable all the actions to be performed.
Erasing external hard drive to GUID partition is only for partitioning and resizing the hard drive, however, it is not an easy job to format MBR external hard drive. Erasing external hard drive will cause data loss. Therefore, you are recommended to download and use Stellar Partition Manager software to perform adding, resizing and deleting of partitions. This application allow you to add, resize and delete partitions even without reformatting or erasing external hard drive to GUID partitions on Mac OS.