Drat! Sounds just like my problem. Here's the text I drafted for this post, before I found the right forum:
SUMMARY: Android File Transfer doesn't believe the phone is connected to the laptop, even though the phone is getting charged from the laptop through the cable.
My MacBookAir6,2 is running High Sierra, MacOS 10.13.1. My Samsung S-6 Verizon SM-G920V is running Android version 7.0, security patch level Nov. 1, 2017.
I just downloaded Android File Transfer.dmg (from
https://www.android.com/filetransfer/) and moved the app to the Applications folder, just as the download page tells me to. The app says it's version 1.0.11, created Friday, October 27, 2017 at 6:15 PM.
I connected the unlocked phone to the laptop with a USB cable.
The phone says:
>>>>>
*Attention*
The connected device is unable to access data on this phone. Reconnect the USB cable and try again.
Android File Transfer must be installed on Mac OS X devices. To download Android File Transfer, go to
https://www.android.com/filetransfer.
<<<<<
This was the third time I downloaded the app (from
https://www.android.com/filetransfer/), and the third time since this download that I've connected the cable — fourth, now: I turned off Power Saving on the Android. Same message.
The download page says
4. Use the USB cable that came with your Android device and connect it to your Mac.
Well, I've had this phone for a while,* and the cable is not the one that came with it... but if I recall correctly, it IS one that I bought at a Verizon store.
*Actually this is a replacement of my original phone, under warranty, because the USB socket apparently got distorted, and I couldn't count on any cable to stay connected unless the phone was absolutely still. I've transferred the data and apps, mostly, and the cable works fine for power, including when it's plugged into one of the laptop's USB ports, like now. It's connected for power, all right, but not for file transfer.
Any suggestions, anyone?