I don't know why but, seems that
1. by default seems that all programs installed via qpkg run with admin
2. there is no sudo so no su command availabe, must be installed via ipkg
3. not all version of qnap have disown

I don't know why but seems that some peoples have some trouble to start jd2, ok during the installation phase if you close the ssh session and jd2 is not on background, but in theory if you go to the cpannel and start it again must works! But seems that is not true, and is strange.

So:
I tried to use screen, after a jd2 update screen wont works
Not all qnap have disown!

I've now 2 way
1. installing sudo via ipkg and run your command
2. installing coreutils via ipkg and run nohup