old data transferred to new computer via time machine, is external hard drive still usable

I set up my new macbook from an old macbook via time machine and an external WD Harddrive.

The setup partitioned my macbook to two user id's, now I need to back up.

The old user id is backing up everything again and the new user id wants to create a new backup to my WD Harddrive like it is not used.

I don't believe that my WD was partitioned.

Help! How can I continue using my WD and backup both user id information without losing anything?