I have a WD My Passport Ultra Portable Hard Drive, USB 3.0, Blue. It works fine in OS X but I find it strange that when I press Alt during startup it doesn't show its OS X volumes. I created 3 volumes with different OS X installs (10.9, 10.10, 10.11) for testing. It's formatted as Mac OS extended journaled. So I don't know why this is happening. Weird! I think this drive has some type of protection? not sure, may this be producing any issues. I haven't filevault enabled of course. Doesn't happen with all macs though, some macs show the bootable volumes good.