I've got the same problem as the OP, but installing the universal adb driver didn't help.
I have a Windows 8.1 64bit laptop and the Verizon version of the M8. I've installed the latest version of HTC Sync manager and when I attach my phone with the USB cable, it shows up in my Windows Device Manager working correctly. When I first plug in my phone, I get the windows sound and 3 windows on the upper right of my laptop asking what I want to do with the phoneq and the CD rom. I've tried choosing different choices and not choosing anything. When I click on "Check for firmware updates" I get a "Yes" answer and after a few minutes the phone appears in the Sync manger window saying "Phone connected Loading data". Then, after about a minute, I get another pop-up saying,"Device unavailable. Please reconnect your device(4)" I have tried different settingPleases on my phone under "USB network settings" mostly "Smart network sharing". Noththing works. Any help would be greatly appreciated. Thank you in advance.