I too have this problem. The only way I have found to mount every time without exception is to use the Motorola Driver Installer and clean the drivers BEFORE i connect and try to mount phone.Has anyone found a reliable method to mount your droid with vista as an operating system. New build vista premium will not mount my droid. Will not recognize or asks for me to load a disk. Yellow exclamation in mass storage drive in device manager. Tried every work around. Debugging on/off. Mount at various times. Drivers/Motorola etc. Worked like a dream in seconds on XP Pro on another office computer. Thanks.
Here is a link to installer.
USB and PC Charging Drivers - Motorola USA
First run Motorola driver installer exe.
Then select clean only and start.
then connect and mount.