About disk quote question

I had installed nexcloud server on 10GB disk ,and use a 4TB external usb disk to store files.
I am confusing, which disk is the disk quota limit set for each user on the server? Is it the disk where nextcloud is installed, or the disk that includes external storage?
How do I limit only the user’s space on the 10GB disk?

I don’t think it is possible at this point to select different quota per user per mount. What I would probably do in your case it to move the complete NC data directory onto the 4TB drive and to leave the 10G for system things only.

You can move your data directory my simply moving it on the file system and by changing the config.php variable accordingly:

$CONFIG = array (
  'datadirectory' => '/4tb_drive/nextcloud',