Format the "Slave" drive.
Depending on your operating system you may not be able to run your old HD with its Windows and your new HD together...... If you can then in My Computer select the slave drive and click on File and then Format.
If you cannot run both drives together at present use the DOS approach
Be aware the old fashioned DOS format is final and can't be undone, so I suggest that to avoid confusion unplug your new HDD and plug in the old.
Boot from a floppy, and at the A:> prompt, type format C: