WD Passport Ultra can't mount on Mac, flashing lights, but can be read on Windows. What is broken?
This happens because OSX "forces" a disk check/repair in the background before mounting the drive (since it was ejected improperly), so it will simply appear to not be mounting until that completes.
It seems it was fixed for you when you let the check complete---
A quick way to "fix" this is to manually force quit the fsck process from activity monitor.