There are a lot of devices which we can use for NAS server e.g. Raspberry Pi
(through USB), or Cubieboard
(through SATA) and also Banana Pi R2
GnuBee Personal Cloud 1 (GB-PC1)
is better for this application. Up to 6 2.5" HDDs (SATA) can be connected to GnuBee.
You can use other peripherals for connection of HDD, 2x USB 2.0, 1x USB 3.0
contains two gigabit interface
The processor of GnuBee is MediaTek MT7621A
The processor can use up to 512 MB DDR3 RAM.
The OS can be included on microSD card (up to 64 GB).
Be careful for power supply, 12V and no less than 3A is needed.
Supported OS are Debian
The GnuBee-PC1 costs $168 (without HDDs).
Link here https://www.crowdsupply.com/gnubee/personal-cloud-1