A Microsoft file hosting and synchronization service.
I don't know why adding the additional 1TB would cause OneDrive to stop syncing. What happened to the files that were in OneDrive before you added the additional 1TB? Did the files move into the recycle bin at onedrive.com? Did you delete them from the recycle bin? They don't just disappear.
Yeah, when they went to the recycle bin I deleted what was in there to ensure that wasn’t messing with free space. The fact this all happened after adding the 1TB is far too coincidental.
I’ve sent screen recordings and pictures to Ms Support, they’re advising me to try all the things I’ve already done, worryingly. The fact I can’t go to OneDrive.live and upload a file because I’ve maxed out storage surely screams an issue with my online account rather than my PC.