Frost on back of refrigerator, left side - Sub-Zero Bottom Freezer Refrigerator

Your evaporator coil has a leak in it and the unit is cooling with minimal freon in the lines. So the only cold part of the coils that has freon is collecting moisture and forming the frost on the inside back wall. You need to contact a qualified service technician to resolve this.

Just frost or a puddle? If it's a puddle cause by melting frost with no where to go then you had my problem. A clogged drain. First defrost the drain, a hair dryer works well, blow or snake it out then install the factory drain heater fix. Plugs right in and works like a charm. I've had my sub zero 550 for more years than I can remember. I'd buy another one in a minute. Good luck!