Some new infos here:
I redeemed my 50gb (which got upgradet to 100gb) Drive storage last april. So far, so good.
2 Days ago, i flashed the ARHD 30.1 Custom Rom, based on the 5.0.1 Lollipop update.
It seemed to somehow changed the device ID, making Google think this is a new device --> Because i again get the drive storage reminder in Blinkfeed.
I have tried to lock my bootlaoder again, and try to redeem it into my account, to have 200 gb (worked on my Galaxy S3 2 years ago, where i got 48gb Dropbox storage. And a second time, after a Rom changed the device id).
However, in HTCs Case, this seems to be limited to one time 100gb per Google Account.
My Theory: That Rom (or even the whole update) changed the ID, and you get 100gb for a 2nd Time. But you need a second Google Account, to redeem it into (one, that has NOT gotten the HTC drive storage yet)
For example a Gift for a Friend?
But this still needs confirmation. Might as well just be a bug or something, but i think my theory is correct here.