Memory Card Reader Unexpectedly Ejects the Media
Your Memory Card Reader unexpectedly ejects the media after 10 minutes of inactivity
The firmware on your card reader may be identifying the reader to Mac OS X has a hard drive. When Mac OS X power management attempts to put this "hard drive" to sleep, this may result in the card reader ejecting any mounted media.
This should prevent the card reader from ejecting the memory card for up to three hours of inactivity.