User profile configuration is taking advantage of the User Profile Disk (UPD) technology in Windows Server remote desktop services. It's a new technology that makes the profile management a lot easier. 

The way it works now is that each user is assigned a virtual hard disk which will hold their profiles, including their home folders (Documents, Downloads, Favorites, Desktop, Appdata, etc.). 

Profile data size 

The maximum size of the drive is 100 GB. The size is a standard we chose so it would give users enough space for all their files and ost/pst files but also give them certain limit so they don’t overuse the storage space. The user profile disk will grow as the user downloads and creates more data on them.
The total storage of the boot disk will need to be increased as the user profile disks occupy more space.

This user profile disk limit can be increased if needed. If not increased, it's thin provisioned so it will not go past the allocated 100 GB size. There is a tool Microsoft published that can help in identifying who the user profile disks belong to and it's called Sidder. 

Moving Data to user’s drive

As far as getting files into users’ drives, you can do it through group policies or by mounting the drives and copying the files you need into the user profile data when logged in as admin in the server.

User profile data location 

In the file server, in C:\Profile folder
If multiple session host servers are created, the profile is still hosted on the file server an is roaming to the session.

Profile folder is created for users when they are sent to cloud

User’s drive security

Profile disk is permanently housed in the file server and will only load for the owner. Anything stored in any of these folders will be safe for the user and there's no need to create individual folders for each user.

User Profile Disks and admin accounts

The UPD is configured for a collection, which really means that it's configured as a computer policy for the servers which are part of that collection, in other words, when it comes to UPD's it's the same setup for a standard user or an admin.

One of the rules for a UPD is that it can only be mounted to 1 session at a time, which means that if you have multiple servers in a collection then you can only really log into 1 of the user session servers at a time, if you log into multiple collection servers at the same time, only the first session which was able to successfully mount the  UPD will be able to use the actual profile, all other sessions will be logged in with temp profiles.

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