If you're running Fedora <= 20 and having trouble with mounting a WD My Book hard drive via USB 3.0, take a look at this bug posting:
https://bugzilla.redhat.com/show_bug.cgi?id=1091513
Following the instructions there to remove the timeout on running ata_id fixed the problem for me - i.e. just comment out line #35 of /usr/lib/udev/rules.d/60-persistent-storage.rules
Looking around this seems to be quite a common issue, but the fix doesn't appear to be well known or properly implemented yet. It's such an obscure but simple fix I thought it was worth sharing.