This note explains how to set up a cheap Raid 1 NAS with an Odroid or Rasbperry board and two USB-harddisks using OpenMediaVault. Only very cheap and largely available hardware components are used. When very high availability is required it is recommended to hold available a second Oroid/Rasbperry board with a mirrored SD-card containing the NAS operating system. In this way the system has no single point of failure.
Update: 20180626: Upgrade to Debian 9 and OVM 4.
Update: 20180624: Troubleshooting section extended.
Update: 20180925: Armbian automates the installation of OVM.
Update: 20181011: Add a section about NAS firewall configuration with UFW.
Update: 20181020: Improve structure and detail some procedures.