I just let Mavericks 10.9.2 install a security update on my current model rMBP. I have the HD partitioned so that there's a small EX-FAT drive that has some work on it that I share with a Parallels Windows VM. It's worked fine for the couple months I've had it this way. Until now... After installing this security update this morning, the drive is no longer mountable by OS X. Disk Utility sees it, but says it's formatted as MS-DOS (FAT), which it's not, or at least wasn't. I can't access the partition. AND IT HAS A LOT OF MY WORK ON IT!!! Most of that, probably all of it that matters, is backed up, but still. My whole setup depends on this working and it's going to take awhile to get this right on my larger Mac partition. Any ideas as to what happened and why and, more importantly, what, if anything, I can do to restore what I had? Argh.