That shiny new HTC One M8 is supposed to come with 50GB of free Google Drive space, but what gives? AT&T is holding out on us. It was actually just a bug that popped up last month, but AT&T promised it would be fixed by mid-May. Here we are right about the middle of May, and the fix is rolling out.
Resolving the issue required a system update to be built and approved by AT&T so it could be delivered over the air to affected devices. The update has version number 1.58.502.1 and should appear on all devices soon. You can check Settings > About > Software Information to see if you're on the new version yet.
When that's done, head into the Google Drive app to see if it registers the free space yet. If not, you might need to clear Google Drive data in the app settings and log in again. The notice has been posted on the HTC Advantage page down near the bottom, if you want to see it with your own eyes.