@NatoBoram, you hopped onto an old thread...
It is quite possible a size issue you are facing. You didn't state the drive size, so I can't tell.
NG Routers do have problems large drives (over 4TB's) and portable drives as well.
I've also seen reports of drives working in one port but not the other.
You might be better off starting a new thread with the drive details and what didn't work and what you did to make it work? Although, since you seem to have 'solved' your problem it probably isn't worth it.