My new Western Digital My Book external drive will not erase to re format for my iMac.
That is to say it starts but then goes no where. No green in the bar. What's up?
With the limited information you have presented, formatting drives with Mac OS 10.0 (2001) to the newest version 10.13 (Dec 2017) has always been the same. I recommend you try mounting any partitions first. If there is data on the drive but your Mac device cannot read it even after attempting to manually mount the drive partitions, then I recommend plugging the drive into a computer with support for the file system being used on those partitions. Ext 1-2-3-4 are all read and writeable by the majority of Linux operating systems, FAT32-ExFAT-NTFS-ReFHS are all read and writeable by most Microsoft Windows operating systems.
Please Note that there are GUI differences between Mac OS 10.0-10.10 and 10.11-Current (10.13 as of writing this).
The other answer does a good job of how to use disk utility to format a normal drive, but not all of WD external drives will work with that. At least their Passport series has a ‘feature’ called Smartware that prevents you from doing anything useful with the drive, even something basic like formatting it. Even zeroing the drive out with dd won’t work.
This like looks like it has instructions that should work to disable it- https://www.reddit.com/r/linux/comments/17b64a/psa_avoid_wd_passport_drives_you_cant_reformat/
If I can find my old passport drive and test it out I’ll update the answer with the necessary steps if it works