I think that people's associations associations for the the word 'zombie' get in the way of dealing with it. They are not supposed to be scary or a scourge on the face of the earth. Just an "oops" kind of user action. We all have had "oops" moments.
Over time dozens of users have asked me to kill zombies. Because they could not kill them. They never like the answer. As in this case.
Zombies are okay. A few at a time. Way too many of them uses up available process slots. Which will ultimately force a production reboot.
giving a result?
If it hangs hard, then there is some serious trouble. Then do further diagnostics, e.g. check mounts with
mount | grep /backup
Not responding to kill -1 does not prove that it is a zombie.
It is a zombie when its state (S column) is Z in
ps -lfp 21758806
Also, most Unixes replace the process args name (CMD column) with the word "defunct".
When it is in D state it is in "device-waiting", not kill-able - but not a zombie.