The RPi, even the 3, isn't a great choice for networked storage, since the NIC is running over the USB bus. A Beaglebone Black[1] would be similar but have better hardware for the job.
I have a Beaglebone Black with a WD USB 2 portable HDD attached to it. I get only around 5 MB/s while copying (from Beagle to PC) a ~500 MB file over 5 GHz wifi. Not sure if this is because of USB 2.0 though.
My understanding is that the Beaglebone Black is also... opener. Or at least it seems like the Beaglebone Black is supported by more operating systems. If, for example, you wanted to run OpenBSD or even Minix 3, BBB is the system for you.