I've been chasing the issue with the sdcard unmounting anytime the device is plugged in via USB, so I wanted a dedicated thread on the matter. I think I have a fix for it now. First, confirm whether or not you are having the problem. Set your phone to default to "Charge Only" connection type, then plug it into your PC. It has to be in that order. Then, use Astro or Root Explorer to see if the sdcard is still mounted on the device. If not, you are encountering the issue I am referencing here. To fix it, launcher ROM Manager and select "Fix Permissions". Then, diisconnect USB and reboot. Run through the testing process again and see if the sdcard is still unmounting. Please report back with your results.