Your message was not entirely clear. Do you mean that
1) You have an external drive (USB? FireWire? USB 3.0?)
2) You have used it to do backups of your iMac
3) You NO LONGER NEED THOSE BACKUPS and can afford to have them lost forever
4) And you want to use the drive with Windows instead, never again with your iMac?
In that case, you can use the Disk Manager to initialize the drive, create a single partition, and format it to NTFS. Any data that was on the drive will be gone.
If the above guesses are wrong, please set us straight.