Immediately after upgrading a 2013 MBP to Yosemite over the weekend, my two external hard drives (one formatted to run Time Machine, the other formatted FAT32 for overflow file storage) After a subsequent reboot, I can no longer see nor mount either drive. OS X Drive Utility shows both drives, as well as the named partitions. When attempting to mount either, I was prompted to verify the drive, which was unsuccessful. I was then prompted to repair the drives, but unsuccessful in this, with an error code 8. I could live without Time Machine for now, but with many photos on the FAT32 drive, I panicked and plugged into a Win7 laptop, and the drive (thankfully) was intact and accessible. I then installed a Windows-based utility to allow reading the Mac-formatted Time Machine drive, and was able to explore contents of that drive as well. Anyone else see these kinds of issues with the upgrade or even earlier versions of OS X?