Drive calculation will only be updated daily as the process to calculate the drive usage is resource intensive and will only be performed at the daily (recommended at non-business hours) scheduled time.
Due to this limitations, the user can still upload files until he/she "exceeds" the assigned drive quota within 1 day. However, on the next day, the user will notice the quota has exceeded and cannot upload any files.
e.g. If user has only 100MB of space left in the drive. Then, the user continues to upload multiple files of size between 1 - 99 MB and all files totaled to more than 100MB. The files can continue to be uploaded. However, if the user decides to upload a 110MB file, the system will prompt that there is no space left for user to upload.