It might be how permissions are setup on the server. Sometimes permissions are not inherited by child folders as you expect. A simple checkmark determines if the attributes are inherited by child objects or not.

On /Portals/0/YourFolder right click select Properties, Security tab, then Advanced button. Depending on the server it will says allow or include inheritable permissions from parent or something to that effect. You can also select the permissions tab and verifiy that Network Service has full control which applies to this folder, sub folders and files. Now compare this to the parent folder /Portals/0 or /Portals.

If Network Service (assuming IIS6/7) does not have access, the easiest way is to fix the root DNN folder using that advanced tab, then select Check allow inheritable permissions AND replace permissions on all child objects, then click ok. From now on the permissions will be inherited automatically.

I hope this is of some help to you.

Turning off the Inherit permission flag is common in a shared hosting environment. The hoster does not want permissions to be inherited from the root of the drive or anyone would have access to the other files. DotNetNuke needs to inherit from it's root folder to creates files and folders.

Using SWAG (Scientific WIld Ass Guessing) I think it is highly unlikely that .net 3.5 file I/O works differently than 2.0, unless it is configured to run under a different context, using different permissions. 

A quick way to check is to login as host and select Host|Host Settings.  Can you list the configuration values here?

Another issue to consider is who made the child folder under what context. Your user account (or a host account) may have proper permissions, but DotNetNuke running under NT Authority/Network Service may not.

When you add an object you specify specific permissions as well as how it applies to the container. The choices are this folder only, the folder subfolder and files, theis folder and subfolders, this folder and files, subfolders and files only, subfolder only files only. This can be very frustrating when some things work and some things don't all because of the settings. Permissions are very easy to mess up if you are not sure of what is required.

The only other thing that I can think of is that the the path and file are formatted properly, and it is exactly what you expect. I would double check to see If DNN File Manager works to create a new child folder, put something in the new folder, delete it, then delete the folder. If that works then the permission for /Portals seems to be correct. That doesn't mean that the permissions are the same for the entire DNN folder structure.

I'm am really interested in your troubles.  Please keep me posted when you find the answer.

Thanks!