If you are using My Passport for mac then you don't have to do anything, as it comes Pre-formatted in Mac OS Extended Journaled file system. Just plug in the drive and drag and drop the files in the drive
But, if you are using My Passport which comes pre-formatted in NTFS files system which is compatible with Windows only, then you have to reformat the drive in Mac OS Extended Journaled file system in order to make it compatible with Mac, and the scheme would be GUID Partition Map.
Also, if you want to use the drive with both Mac and Windows then you have to format the drive in exFAT file system, but if you format the drive in exFAT file system then you will not be able to use the Time Machine (Which is used to create the backup) in Mac.
Try to reboot the Mac and format the drive again you will not face the (Erase Process has failed, click done to continue) error again.
Please refer to the link give below in order to know how to format a WD Hard Drive.