Okay, typed into terminal:
./adb-mac version (This command gives the version of ADB that the Mac is using.)
Android Debug Bridge version 1.0.29 (Terminal returns this answer; ADB needs to be at least version 10.3.1 to properly work and fix the "device offline" error.)
Here are the steps I then followed to fix problem:
1. I unplugged my HTC One from the computer.
2. Downloaded and installed Android Studio from the Android Developers’ site into my Applications folder
3 Updated my Platform Tools, as follows:
Control-clicked on the Android Studio Application and selected “Show Package Contents” from the pulldown menu.
Followed this path to the platform tools: Android Studio/sdk folder/platform-tools folder/adb & fastboot
Option copied both adb and fastboot from platform tools to my user directory ~/Android/HTCOneRoot.
Closed Android Studio
4. Restarted the computer.
5. Launched Terminal and typed cd Android/HTCOneRoot( my user directory where my adb and fastboot are stored)
6. Typed into my terminal the command, “./adb version”
Following this command prompt, Terminal responded with “Android Debug Bridge version 1.0..3.1. (Yes!!)
7. Typed into Terminal, ./adb devices
My device did not show up right away!
8. So, I unplugged my device. Shut down my Mac and restarted it.
9. Relaunched Terminal and typed cd Android/HTCOneRoot
10. Typed ./adb kill-server
11. Then typed, .adb start-server
Terminal responded with: daemon not running. starting it now on port 5037 *
daemon started successfully *
12. Plugged in device again, and the device dialog about RSA appeared. (Whew!)
13. Clicked “OK” on the dialogue box.
14. Now typed ./adb devices and my device's serial number shows up, followed by the word "device" (not "device offline")
14 All done! No more device offline errors.