OK, we'll need some info to help you.
1. What types of drives were these? Both of them older IDE drives, or is one a SATA unit?
2. If they are both IDE drives, are they sharing one ribbon cable and port connector? If so, you need to be sure they each have their jumpes set correctly to establish the Master and Slave functions. Is that done? Or, do you need help there?
3. I got confused when you spoke of not deleting files, and then of wanting to format the drives. A Format operation will destroy all data on that drive. But, if you actually did NOT want to save data on one (or both) of the drives you CAN Format it using the tools already built into your windows OS. No special disk is needed.
4. What is on these drives? If one of them is your C: drive with the Operating System (I presume Windows, maybe not?) on it, you do NOT want to Format that!
Give us some details and we'll advise.