I bought a Lumia 640 from that $99 deal last Xmas and decided to try the 30 day trial for Groove music.
I was unaware of the 100GB free OneDrive storage as there was no mention of it. It wasn't until I was about to cancel my 30 day trial, I noticed the extra 100GB in my OneDrive account. Even after cancelling my subscription the 100GB of free storage remains.
Must have an active OneDrive account to get the 100GB Storage after Groove Music Subscription
If you don't want to be charged $11.99 every month, be sure to cancel your Groove Music subscription before the 30 day trial ends.
Plenty of space to upload your own music collection to OneDrive and play from any device that has Groove Music on it. Loving this feature. Enjoy!!
EDIT:
Your 100Gb bonus storage should show in your Onedrive after 48hrs. Be patient
Microsoft say in order to keep the 100Gb you will have to keep your Groove subscription active. Once you cancel your groove Sub, your 100Gb will also be gone. I've been lucky enough to keep mine after cancelling. You might also get lucky.
Nice, just pray that M$ won't change their mind and cut this down to 100mb after a while