I was given a water damaged iPhone 6, I tried DFU Restoring it and it would error 53 on both my Mac & Windows PC and with different cables & up to date iTunes clients. It had fried itself in the home button cable that continues behind the LCD Shield plate to touch ID/home button assembly connection. I thought I would try getting a new LCD Shield plate that includes the home button cable. I swapped out the LCD Shield plates out and SUCCESS with the DFU Restore! Error 53 in this instance was hardware related! Thanks for the some of the posts here that helped me pinpoint the likely cause which you only discover when trying to restore!