I have two Fortigate devices 1100D in my premises running Active-Passive mode using two port as HA links and one lan and wan link on each devices. Basically I need to move these devices from one building to another.
Need to know what are the precaution I need to take so that my HA should reforms smoothly without any challenges.
Definitively you should fully cable the cluster while both units are powered down*, and then power up one after the other. Sequence and time between do not matter.
If you fully cable both units except for the HA connections (hopefully, you use more than one HA cable), each of the two unit will determine that it is the master. Each will assume identical MAC addresses for the interfaces in use, along with IP addresses as well. This will usually disrupt all network activity.
* to be precise, "while at least one of the units is powered down".