Extending Volume - Force Deletion Warning

I have a Dell Inspiron 1564 which runs Windows 7 Home Premium, Service Pack 1.

My C drive is almost full, so I have started the process to extend that volume. I have duplicated the data from my D volume onto an external HD and am trying to delete the Inspiron D volume; but after confirming that I want to delete, I receive the following warning:

The partition (D:) Simple Volume is currently in use. To force the deletion of this partition, click Yes.

WARNING: Forcing a deletion might cause unexpected errors in the application that is using this partition. Do you want to continue?

I have no idea what is using the D volume, and what the "unexpected" consequences might be of continuing. I did try to remove any association the libraries might have with the data on my D drive by "remove"ing them from the list of Library Locations. Hope that is sufficient.

Is there anything else I need to look into before indicating I want to continue? Is there any way to assess the risk of continuing?

Thanks for any help you can provide.