You should be able to use any functional external drive. It may have to be reformatted using Disk Utility. I assume that you are going to use the Time Machine app to do the backup? The general rule of thumb is that the backup drive should be used only for that purpose and it should be two or three times as large as the data on the HD to be backed up.
If you are planning to use W.D's backup software, I would recommend against it. Experience has shown that the maintenance software provided by the HD vendors is often very problematic. I would only delete it. Use Time Machine for the backup. It's in your computer and it works. I hope this answers your question.