Thanks for your answer @redtomato.
I will google a bit about user space nfs. I'm a bit newbie

I do use DLNA (minidlna) for video streaming, but I need to manage files in the HD connected to mirabox. By now I use sshfs. I prefer nfs or samba, because I want to mount mirabox from fstab in my workstation. Samba seems to me a bit difficult to set up from scratch.
Both external USB's are working fine here.