Likewise mine does not remount the drive - proudly leaves the "you didn't eject the disk properly silly!" message on display. I have up to date AV software Chrisphoto and have had the problem before and after definition and component updates (running Norton Internet Security for Mac).
Interesting finding (so far) is that I have pulled out my main LaCie 3TB (just noticed I said 3GB - duh - showing my age in IT) and replaced this a LaCie 500GB USB drive (USB powered - not separate power supply) and so far all fine. I purposely did not run Caffeine while I left the Mac on all day when I was at work.
So wondering if the ML issue with the random disk eject is related to size or power source of drive (clutching perhaps I know). I have seen others report the same problem with 1TB so suspect it's not a drive size factor.
If the 500GB continues ok for a day or so I will try some other USB drives I have. Just trying to narrow down what the common factors if any may be.
The d2 Quadra 3TB from LaCie is only a month or two old and was not cheap - paid more to get a quality product rather than basic consumer - so I will be rather annoyed if it turns out the problems with LaCie firmware talking to ML rather than with Apple. From some other forums, seems LaCie have not generally been fast to resolve issues with their drives.
jmbell - last night I left my iMac running but just ejected the external drive. Do did not need Caffeine and for me the external is more long term storage rather than used for downloads in the first instance. If the problem persists I may go looking for an apps that keeps the disks spinning but lets the screen go into energy save mode.
Andrew