I upgraded from Mountain Lion to Mavericks today and found that my external ext4 formatted drive would no longer connect; it’s out of my other laptop and was working fine under ML. I tried re-installing fuse-ext2 first but that didn’t work, kept receiving the message that the drive needed to be initialized. Finally, I did the tried-and-true: uninstall fuse-ext2 and osxfuse. I then rebooted and reinstalled both (after making sure the drive was unplugged). Everything works ago so there is some bit of code that Mavericks overwrites when installing.