adb shows devices as offline but system_profiler detects the devices fine
We've been looking for an answer to this for some several months. Unfortunately, the most hopeful response I've seen is this one, which suggests moving the Mac Mini to Linux, whose USB stack is described as 'more forgiving':

https://stackoverflow.com/questions/15727314/resetting-android-device-when-it-goes-to-offline-mode-during-automation

We're going to try shifting to a spare (PC-based) Linux box for the time being; I'll try to remember to report back whether it's more reliable.