Allllllllrighty, so after trolling the internet for a few hours I found this thread to see if it would help solve a similar problem I was having with my iPhone disconnecting while importing photos/videos from an iPhone 6 to the Photos app (and Image Capture as well) using my iMac 27" i7 (mid-2010) running OSX Yosemite 10.10.5.
I tried multiple suggestions in this thread and here's what worked for me for what it's worth:
1) Disconnected Time-Machine backup
2) Restarted the iMac
3) Restarted the iPhone
4) Unplugged ALL USB ports
5) Plugged the iPhone (via Lightning cable that came with the phone) into the USB port that sits right next to the Firewire port (or, the USB port closest to the center of the iMac for those of you who might have newer models than myself)
Boom. Worked like magic. I have no idea what part of this series of steps actually affected the disconnect issue for me, but it did indeed work. Also having tried all the USB ports, I noticed that the USB port closest to the center of the computer was the one that was significantly faster rendering preview images before importing and the only one successful at completing the import.
Holey Moley I hope this helps one of you out, because this was such a pain-in-the-butt issue for me.