Is it possible to share a hard drive connected by USB in a local network?
Thanks to Firelord for pointing me in the right direction, I played around with a SSH Server called SSHelper (it's FOSS and can be found on the play store), and got what I was trying to do.

The website has enough information to get it working, and it is very easy to set a passwordless login.

The only con I see, it is that is rather slow for big file transfers.