On my July 2011 MBA (running 10.7.3), external USB drives randomly become unable to write. This problem has occurred on at least two external USB drives, and the Air has been this way since I got it in August. When it locks, I'm still able to see the drive icon, still able to navigate and open files, but unable to change the finder names or atributes, and unable to save any edits. Rebooting restores writability for an undeterminable amount of time. Attempting to eject the disk gets a disk-in-use error. P.S. I have tried all kinds of fixes with Spotlight--putting the USB drive in the private area, even completely disabling it. No difference.