Vista - Windows Complete PC Backup

I have two hard drives and just yesterday I created a complete backup of my c: drive and saved it onto my d: drive. I went into control panel -> backup and restore center and clicked on the backup computer button and selected my d: drive to save the backup on. it worked great, i even restored my system using that backup earlier today.

since then i've installed a bunch of other apps/games and wanted to update my complete backup. so i went back into the backup and restore center, clicked on back up computer, except now it doesnt give me the option to save the backup on to my 2nd hard drive. i can only select the dvd option. i only want to back up my c drive, on to the d drive. i dont think its a space issue because my c: has 280GB total (220GB free) and d: has 465 GB total (286GB free).