it used to be (back in the day) that google determined if you could upload a file to drive at the beginning of the transfer, and only accounted for it at the end. (i.e. if you started 300 transfers of 3TB all together, they would all complete, assuming nothing else broke their connection).
then they changed their calculation to account for it at the beginning and end. i.e. if you were under the quota limit at the beginning, you could start the transfer, bu the transfer would only be able to be completed if before accounting for that file the size of completed files was under the quota limit.
they now have seemed to change it again.
not only do they account for non completed data (i.e. imagine you're doing 20 transfers in parallel that is 790GB in total (with an average file size of about 40GB). it seems now (based on observation of rclone failing with the quota error) you will have issues completing any file, as it seems it wont let you complete if the transferred amount is over 750GB (even if nothing has been complete).
it seems that it wont let you start a transfer if it will take you over the quota in general. Because I was having issues, I just did a test where I first transferred 749.356G (per rclone) and then tried to transfer a 45GB file. the 45GB instantly failed, in the past it would have succeeded.
just an fyi for those who might be running into quota issues more often now.
i wonder how this will coexist with them letting you upload very large files that are even bigger than the quota. perhaps they either changed that or special case is that to start them, your quota has to be under a specific usage (or 0).