When ever you connect your nexus one or any other android phone to your computer, you would need to manually mount SD card each time you connect your android phone. You can only transfer files to the SD card from computer only when your SD card is mounted, manually mounting it could be really irritating this way.
MountUSB is a free to use software for android phones, which allows you to mount or unmount your SD card just by clicking on the icon. The only thing you need to do it run this application in background all the time.
This application is available on Android Market, you can easily find it with its name and install it for free and then run it, now connect your android phone via USB it will auto mount this time as this application makes it possible.