Please see Answer ID 18670: Format or Partition Drive Error - Cannot Dismount or Unmount Windows & macOS for additional information on this particular issue.

If at any time an error occurs stating that the drive can not be dismounted or unmounted, this is not caused by an issue with the hard drive.

If this is the first time connecting an external device to a macOS, refer to the section titled Operating System Compatibility for help formatting the drive.

Please contact Microsoft or Apple for more support. The partition table is used to display each partition within the Operating System and if this becomes corrupted it will lead to data loss. Western Digital does not recommend multiple partitions due to potential corruption of the Partition table.

Western Digital recommends that when formatting a drive that it be the only drive present on the system to limit occurences of accidental data loss. Once the process begins, ALL THE DATA ON THE DRIVE WILL BE LOST! This process is Data Destructive and cannot be undone. The instructions in the article below are designed to help repartition and format a hard drive. The instructions do not differ when formatting different capacity sizes, this is not capacity specific.Internal drives do not ship pre-formatted and will need to be Partitioned and formatted before being used.Please refer to Answer ID 25242: How To Format an External Drive or Memory Card to Out of Box and OS Compatability. External hard drives come pre-formatted and ready for use.

Reformatting a hard drive for mac