Configuring High Availability

There are a number of different SonicWall Knowledge Base articles about setting up HA (see main links below), but I've always done it a certain way, and it avoids any potential issues - and covers every version.

The later methods will likely work, but this is my recommendation (in brief).

Register both devices (direct on the legacy www.mysonicwall.com portal, or via the new platform.sonicwall.com portal).
Log into each appliance and ensure the admin password is the same on both devices (new firmware forces you to change the default admin password, so ensure both are changed to the same one).
Complete registration from on the firewall so it syncs with the licencing server.
Install the latest firmware on both devices (and reboot).
Make sure the interfaces you are using aren't configured for PortShield - they need to be Unassigned.
Configure the HA settings on the primary unit by enabling Active/Standby (usual method), and enabling 'Stateful Synchronization', Preempt Mode', 'Virtual MAC' and 'Encryption for Control Communication' where applicable. Then enter the serial number of the secondary unit, and select the HA Interfaces (in most cases, we'd expect all but 'Preempt Mode' enabled. Preempt is sometimes used without stateful synchronization to force normal operation through the primary appliance).
Connect them together using 1 or 2 interfaces (I've always used 2x different interfaces when using Stateful, but many use 1). Make sure you use the same on each firewall, i.e. X4 to X4, X5 to X5.
Check HA status for secondary unit to be synced and be ready.
Configure Monitoring so that the non-active firewall has access to the internet (use the LAN interface). Use 2 spare private IP address to allocate to the physical primary and secondary firewall.
*** If not using X0 as the LAN, make sure that you connect the X0 interfaces to each other directly ***

Links to HA KB's:

SonicOS 8 - High Availability Administration Guide (pdf)
SonicOS 7.1 - High Availability Administration Guide (pdf)