I had to re-format my brand new WD “My Passport: Essentials” for Mac, and got an error like “cannot unmount” when trying to use the quick format. I noticed a few people had the same problem, so decided to post the solution.
Go to http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/std_adp.php?p_faqid=3865 and follow the instructions for Mac OSX. The key is to make sure that the WD Smartware is not open. If you can see WD Smartware or anything other than My Passport and your usual stuff when you open your finder close it and then follow the instructions to partition the drive in your disk utility.
Hope it helps.