External harddisk not compatible with Time Machine

check this video out at may help you a little, and understanding it , fast forward in to 1:05 ,the details you need is in there ,

best of luck 🙂

https://www.youtube.com/watch?v=UZsyzzfjwIg

In order to use a HDD for Time Machine it must be formatted to Mac OS Extended (Journaled) (via Disk Utility>Erase).

Ciao.

if you do use Journaled then the hard drive will only works in apple product like macbook Pro ,iMac,macbook air and so on

here watch this video to i think he has the same problem you do, see how he fixes it ,

https://www.youtube.com/watch?v=dWiRKi4PJpU

if you don't have anything on the hard drive then erase it and try to do the back up again it may work

and if you do have things on it then take them out , plug it into the macbook then open a new file in the macbook transfer all of information out into your MacBook makes sure you have all of it, then you can erase it, then transfer all of the information in the macbook back in to the hard drive so you won't lose anything

best of luck 🙂

For a more exhaustive explanation, refer to this site;

http://pondini.org/TM/5.html

Ciao.