If you get a warning then check inside USB port. I had this behavior on GS2 where the tab inside port was touching the inside wall. I used a micro screw driver and slightly lifted it up so its not making contact with the metal housing. Be very careful as you have been warned. If you break the tab means your phone is done. So proceed at your own risk.
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My cellphone journey:
Phones-in use: Nexus 5
Phones-backup: HTC One X, iPhone 3G, HTC Fuze
Tablets: Galaxy Tab S 8.4, LG G Pad 8.3, Kindle HDX (8.9), Dell Venue 8, Archos 605 wifi [160gb]
Devices Sold or Broken or given away: AT&T LG G3 (Sold), S III (i747), GNex (i9250), S II (i9100), SGS Captivate, Nexus 7, iPhone 4, Apple iPad2, Nexus 7 FHD, Archos 80|G9
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