Under Windows 7 it's almost impossible to stop this behavior.
It seems to be related to the fact that Win7 keeps accessing the device even when it has no ligitimate reason to do so.
If you hook up a generic USB drive that has an access indicator LED you will see it flashing at random times after you mount the drive.
The classic reason given for this is that it is happening because the drive is being accessed by background services like system restore, indexing, and windows defender, but the strange thing is, the scanning seems to continue even after these services are disabled.
Sometimes this hidden scanning service can interfere with your ability to eject the drive, so I made a concerted effort to figure out what it was one day, but even after eliminating all the legitimate background services above, it is still happening.
Strange, VERY STRANGE.