Glenda- It is a little unclear what the configuration of your computer is, and all the former UFT gurus are off downloading the latest version of TMG today, but, yes, it should be possible to install UFT in your D drive.
But, no, you can't physically disconnect your C Drive without reformatting your new hard drive, reconnecting it as primary to make it the 'C' drive, and reinstalling Win 95 on it. Not recommended.
If your old C drive is full, then you should probably consider deleting some non- UFT files from it before continuing to get some working room in any event.
Please provide a little more info on the exact status of your two drives, and tell us how UFT got installed on the 'D' drive. Thanks, John