My back fell off as well. I've searched online for a fix, but only found the gasket for the front. Since I have the 1st gen watch, i'm out of warranty. So, I went with a more robust solution instead of hot glue that I've seen some people mention online. Go to amazon and pick up some RTV. Search "Permatex 81158 Black Silicone Adhesive Sealant, 3 oz. Tube" on Amazon.

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Step 1) Buy the RTV (its less than $6)

Step 2) Power off the watch. Remove the watch straps (may not be needed, but I didn't like them flapping around)

Step 3) Using a wooden toothpick (so you dont scratch anything) try and clean up any original adhesive from Apple. Dont worry, it comes off easy... that's why the back fell off remember? haha

Step 4) Using another wooden toothpick, dip into the RTV tube and blot/paint around the inner portion of the circumference of the inner opening. Dont worry if you get a little in the watch as it is a natural insulator

Step 5) Verify the cable from the back of the watch is still properly connected to the main housing and press the back down on and straighten if necessary.

Step 6) Place the watch face down on a flat surface (i used my mouse pad) and then set another flat weight on top of the back (I used an old 3.5" hard drive I had laying around) overnight.

Step 7) Turn on watch and enjoy!

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If anyone is interested in seeing the specs of the RTV, then here ya go:

https://smile.amazon.com/gp/product/B000...

Permatex Black Silicone Adhesive Sealant is a general-purpose, black RTV indoor/outdoor sealant. Effectively bonds, repairs, seals, mends and secures glass, metal, plastics, fabric, vinyl, weather stripping and vinyl tops. Also protects and repairs electrical wiring and may be applied to horizontal or vertical surfaces. Waterproof and flexible with a temperature range of -75 degrees Fahrenheit to 450 degrees Fahrenheit, this sealant is oil, water, weather, vibration and grease resistant. Suggested applications include windshields, door frames, electrical connections, seams and roofs, vinyl tops, and weather stripping.