The M3 is a different little board it works very well for certain tasks.
Using it as network storage for back up would be fine.
But really would not make use of the BPI-M3 advantages.
As for speed of access to a network drive for media streaming of HD or 4K you may be disappointed.
The Ethernet Port and the SATA port are both connected to the SOC by USB2.0 so you will be limited to USB2 speeds.
One thing I have running at my Lab is a compression stack using FFMPEG for the last 66 days of uptime.
I am archiving 24TB of various format movies and down converting them to 720P H264 AVC AAC to use in my truck media system and my iPad/iPhones.
Sitting on the network BPI-M3 is happy to copy the files over to the local 2TB connected to the SATA port then transcode them and copy them back to the larger server.
I have been involved with other projects and have yet to get back to the porting of mainline to the A83T.
An Example of a good use of the A83T is https://dev.mysql.com/doc/refman/5.7/en/mysqld-multi.html
Happy Holidays and Merry Christmas:santa: