While many Mac owners have been dipping their toes into the latest Mac OS update, Mavericks, a few have been having some other strange problems. According to growing complaints from the Apple Support Communities (opens in new tab) forum, some users have found that their Western Digital and LaCie hard drives have been turning up… empty after upgrading to the new OS. In most of these cases, users have configured their drives using third party utilities or manufacturer-provided software like the Western Digital Drive Manager.
To what I'm sure will be the relief of many, the data isn't actually gone, it's just not being accurately reported to the OS. If you've upgraded to Mavericks and find yourself missing some critical data, there are a few things you can do.
First, you should try to create a secondary back-up using an additional computer if you haven't already done so. If something goes wrong down the line, you at least have a back-up for your back-up. Next, check with the manufacturer of your external drive; many of them have updated utilities that you can run that will solve the problem.
If you're still having trouble, you can try to roll back to an earlier version of your OS, but depending upon whether or not you've been using the now non-functional external for Time Machine updates, that may present its own array of challenges.
Apple has yet to officially acknowledge the issue, but chances are good that there will be a fix before too much longer. Until then, keep checking Apple Support Communities, your HDD manufacturer and, of course, Google, to make sure you aren't risking your data unnecessarily.