Wow, so I read through 96 pages of different people stating various problems and solutions to this thread, to see if I could find a nugget that would help me fix my problem. And I feel like most, if not all, of the solutions are speculative at best because Apple has not clearly stated that there is any issue with Mavericks regarding external hard drives no longer being detected. I wish to add my descriptive problem to this graveyard, in hopes that someone out there sees the symptoms and can push me in the right direction.
My problem: I have a Macbook Pro that had Mountain Lion OSX preinstalled, an external 1TB Toshiba USB HD, and an external 1TB WD USB HD, all worked great for well over a year. Installed Mavericks a month ago, then suddenly both hard drives could no longer be detected after the upgrade. However, if I plug either of these hard drives into my Windows 7 OS custom desktop machine, or even my Windows 7 HP laptop machine, I can both read/write to either the Toshiba hard drive or the Western Digital hard drive to this day (still works). When I first got both of the hard drives over a year ago, I had formatted the Toshiba HD using my Macbook Pro (HFS), and I had formatted my WD HD using my Windows HP Laptop (NTFS). I've tried to reset the System Management Controller (SMC) on my Macbook Pro, shutting down completely and restarting, and also ensuring power adapter is connected for power. Both drives refuse to be detected since the Mavericks upgrade on my Macbook Pro on any of the USB ports on my laptop (I have two USB ports, one on each side of the laptop). The USB cables are fine, the hard drives are fine as well (they function on two other machines). This problem sounds like it is regardless of the hard drive manufacturer, and points to the Mavericks OS itself by Apple. Perhaps I forgot to unmount them properly at some point in time, but I refuse to believe that my hard drives are actually unusable and "corrupted", and hope that Apple can submit an official fix for this problem that renders my backup solutions to be useless.