A little late in the game, but a fix none the less, so i wanted to post it. And since this is still popping up on people's phone, i figured it was worth sharing. After a ton of searching and trying all of the options available. I finally found some files on a windows computer that solved this for me.
Download and install HTC File Manager in PC and you can install HTC File Manager 1.0 in your Windows PC and Mac OS. HTC File Manager is developed by Dalvik Developers and listed under Productivity.
I couldn't find the same files on a mac, so i am unsure if they are available, but they can be copied from a windows install. In the directory, c: program files (x86) htc htc sync manager there are several apk files. AndroidHtcSync.apk and four more labeled with numbers after them. I copied all of those apk's to my phone, and tried to install them on my phone through a file explorer, one by one. One installed, the rest could not. But the fact that one installed, was promising. Well, after that install, htc sync manager on my mac, could once again connect to the m8.
So it appears that the AndroidHtcSync2.apk that was on my phone, was corrupted, or otherwise not installed or running correctly. Installing it again, fixed it.
What a hassle and what a sad state that HTC themselves could not figure this one out. Hi all, None of the posted methods worked for me, so I figured I'd post the fix I finally came across for anyone who might be in the same situation.
HTC ships its phones with USB debugging enabled by default. Every time I plugged my M8 into a computer, it would ask if I wanted to trust the computer, even if I had told it to 'always trust'. I figured this might be why neither HTC sync manager nor Android File Transfer could see my phone. So I went into device info, tapped the build number list item a bunch of times to enable the developer menu, and disabled USB debugging. This fixed everything, oddly enough.
Just something else to try for anyone still having issues.
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Use the power of live video to create and share amazing experiences with family and friends, or even build new fans. (Requires mobile data).