Isn’t the data backup onto another server on another site? Or a temporary back up server might be set up?

OK a few things. Why a 5 year check? Your client does have all of the original certificates for the electrical installation in the building, yes? They also have any and all Minor Works Certificates and Electrical Installation Certificates for any and all works and modifications completed since the original install. Yes? Plus all of your maintenance is undertaken by competent contractors, who verify the integrity of their repairs, yes? If not, why not? There is NO statute law requirement that states you must have a 5 year check. Plus if you do feel that an EICR is required, it is not necessary to shut down the whole building. Whilst it can be a very good method of proving service continuity and resilience. A "black building" check can be helpful. What would you do in the event of a 48hr power outage? This is possible. Plus as has been said, it can be utilised to prove your business continuity in the event of power outages. More information needed, but, there is no real reason to complete a thorough EICR on the premises that requires that the whole building is shut down in one fell swoop. IF your clients FM company wants this, then they are incompetent, and are using incompetent people and you need to find a suitably competent FM company or better still a suitably competent electrical contractor, not a cheap one. Surely all of the servers are not on a single final circuit, fed from a single distribution board? Plus they are surely on UPS's? I do find this hard to believe / understand, unless it is down to cost.