As ubiquitous as MEMs accelerometers and such are, it might surprise you to know that light aircraft manufacturers from Cirrus to Cessna, Piper and Beechcraft are only putting one ADHRS unit in each of their typical two-screen systems in light aircraft.
If you lose that single ADHRS (and they do fail), you are going to suddenly find yourself tossed from that beautiful, colorful 3D wonderland of synthetic vision back into the cold, dull darkness of analog dials, a couple of digital tape gauges and 2D GPS mapping.
Presume your EFIS is displaying little more than red Xs, since the ADHRS just failed.
Or, take the time while the instrument panel is torn apart and wire a second ADHRS into your primary system.
Aspen Avionics and Avidyne will be happy to sell you a dual ADHRS system.
Many non-TSO'd EFIS manufacturers, from Dynon to GRT Avionics, MGL and beyond offer dual independent ADHRS systems with independent battery backup sources.
If you rent aircraft and fly behind EFIS or analog instruments in IFR, you still can avail yourself of several different independent portable ADHRS technologies that have recently come to market.
At its simplest, an ADHRS or AHRS is little more than some microelectronic accelerometers coupled to a magnetometer and the aircraft's pitot-static system.
My AMT had been careful to make sure they were properly aligned with the ADHRS mounting (all were behind the baggage compartment bulkhead).
I didn't even get an error code for the disagreeing magnetometers attached to the two separate but conversational (between each other) ADHRS units.
And all of them must be aligned precisely with the ADHRS computers in order to work properly.
We did worry that perhaps one of our EFIS' ADHRS computers might be acting up, however, the diagnostics didn't pan out there, either.