no the public folder is on diks1 that will have max size 2TB dependent if you define other shared folders.
You can define shared folder in the file share menu
There you can create a new share and choose on which disk it should be made.
When accessing it with explorer you only see the shared folder name and not on which disk it is made. if you want to see that then you should use shared folder names like disk2-abacdef if you make one on disk2 for example
Also you should create users and you can create read/write right per folder per user. The shared folders can be of type public or private. When of type public anyone can access them, if they are of type private only users that have read or read and write right can access the data in that folder.
The shared folder "public" on disk1 is of type public. You might want to copy your data to folders that are private and only accessible by you or users you allow access. Its up to you.
Every window of the webinterface has a help text that can be shown by using the help button in the vertical menu or top right help button