Apple Watch passed on to me from my brother
If I erase all content on the watch so I can pair my phone to the watch will it erase all content on the iPhone it used to be hooked up to
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Make sure your brother has done all these actions, before you try to set up the watch on your iPhone --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
And Ferd II was correct in that it won't erase anything on his watch.
