System Backup - Drive Selection Problem

I've been using Vista Ultimate for some time now and have frequently run the
system backup operation to create a drive image. I have 3 hard drives
installed and have been traditionally backing my C: drive (system) to my d:
drive (applications).

Recently I have noticed that I am no longer given the d: drive as a
destination option for the drive image.

After a little digging I noticed that both my c: and d: drive are selected
as drives to be backed up - therefore, I assume this is wh the d: drive is no
longer presented as a destination for the backup (makes sense).

Since I do not want the d: drive backed up (i.e. I jsut want the c: drive
backed up), I tried de-selecting the d: drive from the drives to be
backed-up; however, it is greyed out...

I really can't think of any big changes I've made to have created this
situation... Any idea how I can get things back to normal - i.e. only the c:
drive is selected for backup?

Thanks