Ice Build up on freezer floor then leaks out the front on to the floor. Drain tube is not plugged. The ice builds up in the drain trough then over flows on to the floor of freezer.

I suppose the ice is building up in the drain trough under the evaporator (inside the freezer). There are two ways used to keep the trough from ice build up. It either has a heater built into it, or the defrost heater is close enough to the trough so that when the refer goes into defrost, it would melt any ice in the drain trough. Or the hose / pipe running from the trough to the drain pan it clogged, in which case you should be able to blow it clean. Note: this hose or pipe has to have a trap similar to the one under your kitchen sink. That is to prevent air from moving up, bringing undesired moisture into the fereezer.

The drain heater goes into the drain,not outside it.
You need a longer drain probe to push all the way through the drain.
Look underneath for a kink in the drain tube.

HI Mark,
The evaporator drain is blocked causing the defrost water to pool, freeze and then leak onto your floor.
Clear out the freezer, then use a hammer to break up the ice on the floor. Now remove the rear panel. You'll see more ice build up. Don't use the hammer on it! (Too close to the coils for comfort) Use a hair dryer to warm up/melt the ice below the coils.
You'll see the drain hole now. Clear the drain by forcing a solution of water and bleach or water and baking soda into it with a basting syringe. If the solution will not flow through, slide a length of ¼ inch flexible tubing into the drain tube to push the clog through to the drain pan. Remove the tubing after clearing the clog.

SG 

I had the same problem but fixed it recently. The problem originates with the drain trough mounted on the back interior wall of the fridge that slopes down into the middle. The drain tube is attached at this point and runs to the pan under the unit. This drain trough is screwed to the interior and sealed with silicone caulking. Over time the caulking breaks free from the interior wall of the fridge allowing the defrosted water to leak past the trough, collecting in the bottom of the compartment under the veggie drawer. It freezes there building up an ice ***** that eventuallly wraps around the drain tube. Any water that now runs into the tube freezes in place over time forming an ice plug preventing any drainage at all. Now the defrosted water begins to flood the interior of the fridge eventually leaking out which it did in our situtation.

There are two solutions. One is to take off the drain trough, clean it up and the interior wall of the fridge. Also thaw out the ice plug in the drain tube and make sure it is clean and drains freely. I used hot water and a syringe to inject it into the tube until the ice melted. Dry everything off. Reattach the drain trough using silicone caulking approved for kitchen/bath use. You may have to shut down the fridge and warm it up to meet the application temperature limit of the silicpne. The other solution is simpler. After cleaning and drying the drain trough, I applied an adhesive backed closed cell foam rubber gasket (3/16" thick automotive) to the back surface of the trough that sits against the interior wall of the fridge when in place. When screwed back in place, the gasket is compressed preventing water from leaking past. The fridge does not have to be warmed up for this fix. It has been in place for a few months now with now problems.

Check the ice maker. sometimes they get cracked a will leak water into the freezer compartment. Also check the ice maker fill tube for leaks or someone could have spilled something. Although this is tipical of a clogged drain. What happens is when the fridge go into derfost, it will melt all the frost that is on the evaporator, turning it into water. The water drains down to a trough under the evaporator and down a drain hole in the trough ( this trough is behind the evaporator cover ) Removing the melted frost from the freezer. If that drain is clogged, the water cannot leave the freezer. Then when the fridge starts back up ,it will freez the water that was not drained. After repeated defrosting the water will fill this trough and run off into the bottom of the freezer to be frozzen when the fridge starts to cool again.

sounds like you have a clogged defrost drain tube. removing back panel of freezer will give you acess to drain trough, melt away the ice with a hair dryer, youll find a drain hole under there somwhere,pouring boiling water down the drain will clear it pretty quickly( i use a steam machine purchased from the infomercials works awesome, after you clear it and all water drain down tube wrap a piece of solid copper wirearound the heater( black calrod heater right above the drain trough) and stick it in the whole maybe an inch or so. now everythime the heater comes on to defrost it will prevent the hole from freezing over

sounds like you have a clogged defrost drain tube. removing back panel of freezer will give you acess to drain trough, melt away the ice with a hair dryer, youll find a drain hole under there somwhere,pouring boiling water down the drain will clear it pretty quickly( i use a steam machine purchased from the infomercials works awesome, after you clear it and all water drain down tube wrap a piece of solid copper wirearound the heater( black calrod heater right above the drain trough) and stick it in the whole maybe an inch or so. now everythime the heater comes on to defrost it will prevent the hole from freezing over

did you try to remove the 2 ***** in freezer floor???. if not remove the floor and check for ice build up underthere, if so:

you have a clogged defrost drain tube. removing back panel of freezer will give you acess to drain trough, melt away the ice with a hair dryer, youll find a drain hole under there somwhere,pouring boiling water down the drain will clear it pretty quickly( i use a steam machine purchased from the infomercials works awesome, after you clear it and all water drain down tube wrap a piece of solid copper wirearound the heater( black calrod heater right above the drain trough) and stick it in the whole maybe an inch or so. now everythime the heater comes on to defrost it will prevent the hole from freezing over

unplug it again , sounds like your drain line has frozen, instaead of defrost water going down the tube and under refrig in drain pan, becuse tube is frozen its draining into freezer compartment and eventually spilling out through front of unit. totally defrost unit and behind back wall of freezer youll see a metal trough designed to catch water, in the center (under the ice is gonna be the drain hole, pour boiling water down until it runs freely

I had a similar issue that started about a year ago with my Sub-Zero 550. My freezer gasket actually tore after several times of freezing closed. I thought that a new gasket would solve it, but a second but related problem developed where I was getting alot of condensation under the left side of the unit, with the water dripping onto the floor...eventually staining the hardwood floor in front of the unit.

I traced the problem to a clogged condensation drain pipe in the refrigerator. It is a corrugated pipe that drains a trough behind the crisper drawers. My pipe was clogged with black sludge and frozen down below. The condensation in the fridge would overflow out of the trough and leak below the crisper drawers, down out through the gasket.

To resolve, I used a piece of solid thin wire folded in half and twisted with a small loop at the end to fish out as much sludge as I could. Then squirted hot water into the pipe until it totally unclogged. I believe this will solve the problem without any need for a heater underneath the unit. The 550 worked fine for 4-5 years before this happened.