You say the rear rotor was being 'ground' ? Do you mean metal on metal contact ? Or do you mean that the brakes were not releasing and the friction material on the pads was in constant contact with the brake disc / rotor ?
You managed to ride it home, so the wheel wasn't completely locked up.
Before solving a problem with the rear brakes, it helps to know what all of the symptoms are. It is true that an SMC that isn't functioning as it should will cause problems on the rear brake, but there are many other things that will cause problems without the SMC being the culprit. And some of these are easy to check and fix.
A few examples:
- Stuck caliper pistons
- Incorrect location of, or damage to pad spring (very possible).
- Caliper sliders not allowing movement when brakes are applied and released (common)
- Too much grease in caliper slider pins (yes, really).
- Rusty / pitted / bent pad spring - preventing movement of pads (common).
- Riding with new boots (and inadvertently applying pressure to rear brake pedal)
- Collapsed rear wheel or rear flange bearings (putting rear wheel and rotor at an angle in relation to the caliper)
- Both pads fitted to the same side of the disc rotor. (Yes, really).
- Pads not seated in the clip at the deep end of the caliper (and resting / scraping on the wheel hub).
- Spacing collar not put on axle (yikes)
- New wheel bearings not inserted in the correct manner (displaces the centre line of the wheel and may cause the disc rotor to scrape on the caliper bracket).
- Caliper bracket stopper bolt not located correctly (unlikely, but possible)
- Front brake pads not releasing properly (thereby applying SMC and applying rear brake - So check pad pins, pad springs, caliper able to move on slider pins)
- Front wheel not inserted correctly - check axle end and clearance between rotor and caliper bracket; also distance collars on each side are correctly installed.
- Front pads are new and too thick, or have a heat shield fitted (there isn't enough room when pads are fairly new - the brakes drag and apply the SMC).
- Compensation port in front brake lever master cylinder blocked.
I could go on (and usually do, but I'll stop there), but you get the idea ? - There are too many scenarios to work out what could be going wrong. Some photos of the damage and of the rear wheel / caliper might help. It might be something simple.
A useful check for the SMC is to check that the rear wheel isn't dragging, and then lie on the floor alongside the left side of the bike, with the bike on centre stand and gear in neutral. Turn the rear wheel with your foot. Apply the SMC with your hand - push it towards the fork to compress the plunger. It moves only a small fraction 1mm or 2, and shouldn't require force (I use the fork leg to squeeze it in my hand like a brake lever). If the rear wheel cannot be turned, then the SMC is applying the rear brake OK. Release the SMC and the rear wheel should turn OK. Do it a few times. If the wheel releases each time, then the SMC isn't the primary culprit. It isn't completely off the hook, but there must be something else causing the drag.