Why can’t you use the D: drive? Is there an error or something?
Anyway, you can use a program like partition magic to eliminate the partition without losing your data from the C drive. I’m pretty sure you will lose the data from the D. The original restore DVD/CD that came with (or you had to make) the TR will also allow you to remove the partition but I am not sure if it deletes the data or not, it’s been so long I really don’t remember.
Erik