Time Machine external drive

Is it possible to TM an external drive?

My Macbook Pro drive failed and Genius Bar replaced it. I recovered the data drive with DriveGenius (it hadn't failed completely, it had slow writes) such that the original drive is cloned to a brand new drive, presumably suitable for direct replacement in the Macbook Pro. However, I'd rather TM the external drive (step 1) to a third external drive and later let TM overwrite the Genius Bar drive (step 2) in the Macbook. By doing it this way, it saves the trouble of opening the Macbook (and the warranty issue) as well as making the TM backup I should've had in the first place.

Is it possible? If so, how? It appears TM will only save the main internal drive and I see no facility to TM external drives? Thanks

Try to read some information in this article: Does Time Machine backup external drives? | One Digital Life

However, I believe that your external drive needs to be formatted as HFS+ (Mac format) in order to be backed up by Time Machine. I'm not 100% sure, but that's what some comments said. That being said, Windows cannot read HFS+ format drives, therefore you'll have to use it only with Mac if so.