I had this problem as well, and the only solution I've discovered is simple, but annoying. In System Preferences - Energy Saver, I had to enable OS X to put Hard Drives to sleep when possible (Checked the box).
The problem stemmed from my external drive spinning itself down after 5 minutes idle. Without allowing OS X to put my internal drive to sleep when possible, OS X was not playing well with my asleep external drive (as it doesn't expect drives to be asleep, I guess). Once asleep, my external drive became Read Only, as I could open files etc., but not copy, move, or delete files (No Writes). I couldn't even unmount the volume without "unplugging" the USB cable, triggering an error message.
With the Hard Drive asleep option enabled, OS X now plays well with my external drive. I wish I could disable that drive from sleeping, but obviously cannot (without "manually" preventing sleep through continued use, I guess).
As I run 10.8.2 on a 2.7Ghz/16Gb/768Gb Retina MBP, the performance hit due to enabling the HD sleep option is likely minimal (though I haven't actually investigated the issue), but still....
I guess if it becomes noticeable, I'll go back to pulling the cord on my external drive, but I don't want to damage it.
Hope this works for those with a similar problem.