XG 430 HA firewall dead

One of the 2 firewalls in my HA pair died. 
when I cycle the power it starts up and stays on before going dead after around 1 min 30 secs. 
Has anyone experienced this before. 
Sophos support have RMA this device and will send a new one. 
Any tips on swapping out the dead one and replacing with the new replacement. 

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[edited by: emmosophos at 9:05 PM (GMT -7) on 28 Sep 2021]