Mac Finder: WD My Passport won't mount

Try mounting the volume at root level with terminal. You may also want to check the integrity/health of the drive with tools such as Drive Genius. I would just do a data rescue on the drive and reformat the it to save time, provided you have the tools and storage to hold your recovered data.

I just debugged a practically identical issue. After hours of trying various stuff, I finally found an answer, after closely examining system.log in Console. I found this:

3.1.13. 21.42.42,090 sudo[622]:    ... : TTY=ttys001 ; PWD=/Users/...; USER=root ; COMMAND=/usr/sbin/diskutil mount readOnly /dev/disk2s2
3.1.13. 21.42.42,000 kernel[0]: jnl: disk2s2: is_clean: journal magic is bad (0xdead00ab != 0x4a4e4c78)
3.1.13. 21.42.42,000 kernel[0]: hfs: late journal init: volume on disk2s2 is read-only and journal is dirty.  Can not mount volume.
3.1.13. 21.42.42,000 kernel[0]: hfs_mounthfsplus: hfs_late_journal_init returned (22)
3.1.13. 21.42.42,000 kernel[0]: hfs_mounthfsplus: encountered errorr (22)
3.1.13. 21.42.42,000 kernel[0]: hfs_mountfs: encountered failure 22 
3.1.13. 21.42.42,000 kernel[0]: hfs_mount: hfs_mountfs returned 22

The issue seemed to be with bad journal. So I disabled journaling and the mount then worked just fine. Afterwards I enabled journaling again, all fine. I'm surprised that Disk Utility was not able to detect this during Verify Disk (which reported that all is fine).

Terminal option to disable journaling is:

sudo diskutil disableJournal force /dev/diskXsX

with diskXsX replaced to whatever is appropriate for your case. This command above will report error at exit, but just ignore it.