Default Phone is looping after update

A couple weeks ago when my phone was running jb I used Odin, the cf auto run file to root my phone and had no problems at all. After the kik Kat update came out I used Odin on my computer and flashed the original firmware so that I could get the update, I knew I would lose root but now when I try to root it my phone goes into a loop. The only thing I could do to fix it was flash a twrp img. file which got it working again. Can anyone tell me what I'm doing wrong this time around and why it's going into a loop? I would also like to know why you can't write to the external sd card anymore???

Sent from my SM-N900T